JP3073777B2 - Home telephone exchange system - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a home telephone exchange system which enables external and internal calls using commercially available telephones.

[0002]

2. Description of the Related Art A home telephone switching system of this type is connected to a telephone exchange connected to an external line, which is a public telephone line, and to the telephone exchange via an internal line as a communication line and a control line as a signal transmission line. A commercially available telephone is constituted by a plurality of telephone outlets that are detachably connected. The telephone exchange switches and sets a communication path between an external line and an internal line or between internal lines, and a telephone connected to the telephone outlet connects an external line and an internal line. Some have made it possible to make calls.
In this type of home telephone exchange system, a door phone is connected to a telephone exchange via a telephone line, and a door phone call can be made using a telephone.

[0003]

[SUMMARY OF THE INVENTION Incidentally, in this type of home telephone switching system, for the detection of the hook state and dial pulse (DP) signal of the telephone C connected to the extension L _1, Figure 4 Hook / DP detection circuit 2 shown
Was used. This hook · DP detection circuit 2, and insert the light emitting diodes Da, Db is a photocoupler PC connected in antiparallel extension L _1, has detected a hook status and DP signals in a state subjected to insulation. Here, the reason why the photocoupler PC is used is that the external line L
_This is because the power supply between the terminal ₀ and the terminal equipment, which is a telephone exchange, must be electrically uncoupled.

When the hook / DP detection circuit 2 is used, the light emitting diodes Da,
Since Db has an on-resistance of about 50Ω, the relay contact r
Is switched to the outside line L ₀ side, the light emitting diodes Da, Da are connected between the outside line L ₀ and the telephone C.
Db is connected in series, and there is a problem that the speech sound is attenuated by the ON resistance of the light emitting diodes Da and Db.

[0005] Also, as described above,₀Directly connected to
Extension L₁When the hook / DP detection circuit 2 is inserted into
Prevents damage to photocoupler PC due to induced lightning surge
It is necessary to invite light emitting diodes Da and Db
Surge absorbing element 40 such as ZNR that absorbs lightning surge
Need to be connected in parallel. Also, a photo coupler PC
In order to adjust the current detection sensitivity of the₁Parallel connection
To remove noise components in the DP signal.
Capacitor C₁Must be connected in parallel. Therefore,
The configuration of the lock / DP detection circuit 2 becomes complicated,
Extension L to be connected ₁The more the number, the higher the cost
There was a problem that it was expensive.

SUMMARY OF THE INVENTION The present invention has been made in view of the above points, and an object of the present invention is to provide a hook / DP detection circuit that does not attenuate a passing sound and that a hook / DP
It is an object of the present invention to provide a home telephone exchange system in which a detection circuit can be formed easily and inexpensively.

[0007]

In order to achieve the above object, according to the present invention, a call current supply circuit for supplying a call current to a telephone while an extension is disconnected from an outside line, and a power supply is supplied to the call current supply circuit from the power supply. A hook / DP detection circuit for detecting a supplied current to detect a hook state of the telephone and a dial pulse (DP) signal.

The hook / DP detection circuit includes, for example, a detection resistor for detecting a current supplied from a power supply to a call current supply circuit, a transistor which is turned on / off by a voltage drop of the detection resistor, and a voltage applied via the transistor. And a voltage dividing resistor that divides the applied voltage.

[0009]

The present invention has the above-described structure,
A hook / DP detection circuit can be provided on a path that is not directly connected to an external line, and a hook / D
If a hook / DP detection circuit is inserted in a path that is not directly connected to an external line, the use of a photocoupler is not required, and the inductive communication is not attenuated as in the case where a P detection circuit is provided. This eliminates the need for a circuit for absorbing a lightning surge, simplifies the circuit configuration, and reduces costs.

[0010]

1 to 3 show one embodiment of the present invention. Real truth
The home telephone exchange system of the embodiment, as shown in FIG.
Outside line that is a public telephone line via the disconnector G and the protector H
L ₀Telephone exchange A connected to the telephone exchange A
Extension L as call line₁And control lines as signal transmission lines
L_TwoAnd a commercially available telephone C is detachably connected.
And a plurality of telephone outlets B
Outside line L at exchange A₀And extension L₁Or extension L₁Communication between each other
The talk path is switched and the telephone connected to telephone outlet B is connected.
External call and internal call can be made on phone C
You. In this home telephone exchange system, the communication line L
_ThreeTo connect the doorphone D to the telephone exchange A and use the telephone C
And can also make door phone calls. Also book
In the case of the embodiment, for example, even in a house of 4DK to 6DK,
Telephone outlet B can be installed in the room by prior wiring
For example, up to six telephones C can be connected.

Each telephone outlet B is provided with a modular jack (not shown) so that a general commercial telephone C can be detachably connected. Also, the telephone outlet B is a speaker for reproducing a broadcast signal, such as the extension ring tone or intercom call sound transmitted through the control line L _2, the voltage sent through the control line L ₂ There is provided a light emitting display unit for discriminating a display signal, which is a signal, and displaying a signal such as during an internal call or external call, and the speaker and the light emitting display perform notification and display operations.

Hereinafter, the configuration of the telephone exchange A of the home telephone exchange system having the above-described basic configuration will be briefly described together with the operation. In telephone exchanges A, as shown in FIG. 2, the relay Ry is provided for each extension L ₁ of the telephone outlet B is connected, make the connection disconnection to the extension L ₁ and external L ₀ in contact r of this relay Ry, supplying a speech current for the extension call or intercom call in a state in which the extension L ₁ is disconnected from the external line L ₀ from speech current supply circuit 4. Note that the contact point r of the relay Ry usually has been switched to the side to separate the extension L ₁ from an external L _0, external L ₀ outside line incoming call, etc.
Side.

Assuming that an incoming call is detected by the incoming call detection circuit 5, the control circuit 11 constituted by the CPU causes the DC connection circuit 3 to perform DC connection and captures the line. By ringing the built-in bell, a call operation when an outside line is received is performed. However, in a home telephone exchange system having six telephones C as in the present embodiment, all telephones C
The do not go to the translation to sound the built-in bell in response to the incoming signal from the telephone station connected to the outside line L _0. Therefore, with a pseudo incoming signal generation circuit 20 for generating the incoming signal identical pseudo incoming signal and sends a pseudo call signal from the pseudo incoming signal generation circuit 20 to the extension L ₁ of each phone C is connected,
Ring the built-in bell. Then, when the receiver of the telephone C is picked up in response to the external line call, the control circuit 11 performs operation control so that the telephone C can perform an external telephone call. Here, the hook / DP detection circuit 2 (where DP is a dial pulse) detects that the receiver of the telephone set C is picked up in response to the outside line call. When handset and is taken up phone C is detected to have been off-hook, the control circuit 11 performs switching of the contacts r and drives the corresponding relay Ry, connects the extension L ₁ external line L _0.
This enables external line communication. Incidentally, the extension L ₁ of the phone C is connected the DC閉結circuit 3 when it is connected to the external line L ₀ to cancel the DC閉結state. Further, the hook / DP detection circuit 2 outputs the DP input from the telephone C.
It also has a function of detecting a dial signal by a signal.

When an outside line is to be transmitted from the telephone C, the receiver is picked up and a dial operation is performed. In the following description, a case where the telephone line contract is a dial pulse line will be described. The off-hook state of the telephone C is detected by the hook / DP detection circuit 2 and the control circuit 1
1 drives a pseudo dial tone generating circuit 21 to generate a pseudo dial tone similar to the dial tone (two tone (400 Hz)) sent from the telephone office, and this pseudo dial tone is off-hook through the analog switch network 10. and it sends to the extension L ₁ that telephone C is connected. However, the analog switches indicated by hatched squares also relate to the setting of the pseudo-sound-tone transmission path and the switching of other communication paths and signal paths. However, the description of the operation of these analog switches is omitted in the following description. I do. Further, the pseudo dial tone from the pseudo dial tone generating circuit 21 is output through a filter circuit 23 that transmits a signal of 400 Hz. The transmission of the pseudo dial tone stops when the hook / DP detection circuit 2 detects the first dial signal.

By the way, in this type of home telephone exchange system, a telephone call processing such as an extension call is performed by inputting a specific dial signal from the telephone C. Therefore, even if a dial operation is performed on the telephone C, an external call is made. There is not always a case, and there is a case of a call process such as an extension call. Therefore, the control circuit 11 having received the dial signal performs the outside line transmitting operation only when the number of digits of the dial signal becomes equal to or more than the number of digits of the dial signal allocated for the call processing (two digits in the present embodiment). To start. The external line transmission is performed by sending a DP signal to the external line L ₀ by driving the DP transmitting circuit 6. When the other party answers in response to this outside line transmission, the polarity of the outside line L ₀ is inverted,
The polarity inversion is detected at the polarity inversion detection circuit 16, then switches the contact point r so telephone was outside line L ₀ and the outside line C connects the extension L ₁ connected, to allow external call .

Here, when the outside line is originated as described above, the called party may be busy. Therefore, in the home telephone switching system, a dial tone transmitted through the external line L _0, for example, with an outside line dial tone detection circuit 8 for detecting a busy tone or the like, it is busy in the outside line dial tone detection circuit 8 If this is detected, the control circuit 11 operates the signal sound generation circuit 22 to generate a busy sound (busy sound),
This busy tone is sent to telephone C that has made an outside line call.

When making an extension call, a dial signal is input from the telephone C in the same manner as in the case of the above-described outside call. In addition, that this is an extension outgoing determines the control circuit 11 from the number of digits of dial signals, by driving the audio signal generating circuit 22 at this time, the telephone through a control line L ₂ corresponding to the telephone C of the called A ringing signal is sent to the outlet B, and an extension ringing tone sounds from the speaker of the telephone outlet B. The hook / DP detection circuit 2 detects that the called telephone C has gone off-hook in response, and the control circuit 11 controls the analog switch network 10 to control the extension L corresponding to the calling telephone C. ₁ and made to perform the connection between the extension L ₁ correspond to a telephone C of the called party, to allow extension call. In addition, when the external call and the internal call are performed as described above, the control circuit 11
The display control circuit 18 controls the light emitting display section to display various indications such as during an internal call or external call. Also,
The circuit shown by a broken line frame i in Fig. 2 each extension L ₁ and the control line L
It is provided in common for every _two . The phototriac FT in the circuit within the dashed-line frame A controls the transmission of a pseudo incoming signal, and the amplifier circuit 29 amplifies the audio signal transmitted to the telephone outlet B.

The above-mentioned extension call is for a specific telephone C, but if the telephone C to be called is unspecified, a general call can be performed. In this case, the dial signal corresponding to the paging
From the telephone outlets B, an internal ringing tone sounds simultaneously, and when any of the telephones C goes off-hook in response to this call, the control circuit 11
It receives the detection output of the hook · DP detection circuit 2 corresponding to, and connects the extension L ₁ corresponding to the phone C extension L ₁ and the calling side corresponding to the phone C, to allow extension call.

Further, the home telephone exchange system includes a hold sound transmitting circuit 9 for performing a hold operation and transmitting a hold melody sound. When the telephone line contract is a push-button line, the push-button telephone C
A dial signal based on a PB (push button) signal from the PB receiver 12 is detected by the PB receiver 12, and the dial signal is transmitted from the PB dialer 7 to the outside line L ₀ while the line is closed by the DC closing circuit 3. Thus, the operation is the same as that of the dial pulse line. The switching between DP and PB is performed by the setting unit 26.

Next, a case where a door phone call is made will be described. In the case of the present embodiment, two intercoms D can be connected, and the intercom monitoring and control circuit 14 detects that a push button of the intercom D is pressed, and receives the detection output to control the control circuit 11. Is the audio signal generation circuit 22
Sends a call signal corresponding to a door phone call to each telephone outlet B, and causes the telephone outlet B to sound a door phone ringing tone. When handset of any telephone C in accordance with the intercom call is picked up, the control circuit 11 and the extension L ₁ of the phone C in response a call line L ₃ doorphone D was performed calls, the analog switch network 10 The connection is established by switching, and a door phone call is made possible. Note that the door phone call is performed via the door phone call circuit 15. In addition, as the door phone D, a door phone D ′ with a monitor camera for photographing a visitor shown in FIG. 3 and monitoring the visitor from inside can be connected. In this case, the visitor is confirmed by a monitor TV unit I provided indoors. it can.

Hereinafter, other functions of the home telephone exchange system will be described. In this home telephone exchange system, an emergency push button J or an alarm unit K can be connected, and when the emergency push button J is pressed or an alarm signal is input from the alarm unit K and detected by the alarm signal detection circuit 24. In this case, the control circuit 11 generates an alarm sound in the audio signal generation circuit 22, sends the alarm sound to each telephone outlet B to generate an alarm, and outputs the output of the alarm sound amplified by the alarm sound amplifier circuit 25. The alarm is transmitted to the doorphone D so that an alarm can be issued from the doorphone D to the outside. In this embodiment, an alarm is issued only from one doorphone D, but an alarm may be issued from both doorphones D.

Each of the electric locks N provided on the door at the entrance where the doorphone D is installed can be connected, and the telephone C which is talking during the doorphone call can specify the electric lock N for unlocking. When the dial signal is input, the relay R is driven to unlock each electric lock N. In this way, the home telephone exchange system of the present embodiment can be used not only for ordinary houses but also for condominiums and the like.

Further, in the case of the present embodiment, a specific extension (in the following description, each of the extensions a to
Explanation is given as f. Then, in the case of this embodiment, the facsimile can be connected to this specific extension as extension f). When a facsimile is connected in this manner, a setting indicating that a facsimile is connected to the extension f is made in the setting unit 26 (for example, a switch is turned on).

When a facsimile is connected, the control circuit 11 gives a priority to the telephone C to perform a call operation for an incoming call at home when the incoming call is to a facsimile, and when the incoming call is to a facsimile, the extension a to e of the received extension. A specific dial signal is input from the telephone C (the same as the operation for the extension call described above), and the call state is transferred to the facsimile of the extension f. Note that, as described above, when the user is at home, in order to give priority to the incoming call by the telephone C corresponding to the extension circuits a to e, a pseudo call signal is not transmitted to the extension f to which the facsimile is connected when an external line is received. .

More specifically, when a facsimile is connected to extension f and there is an incoming call at home,
The control circuit 11 first sends a pseudo incoming signal to the telephone set C corresponding to the extensions a to e. As a result, the built-in bell of each telephone C sounds to notify that there is an incoming call from outside. If any of the telephones C is off-hook in response to this call and the incoming call corresponds to facsimile, the user can recognize from the ringtone that the incoming call is for facsimile. Therefore, if the incoming call is to a facsimile, a dial signal corresponding to the extension f to which the facsimile is connected is input. Control circuit 1 receiving this dial signal
1 generates a pseudo-call signal from the pseudo-call signal generation circuit 20 and sends it to the extension f to cause the facsimile to automatically respond;
As a result, data can be received by facsimile.

When a facsimile is not connected to the extension f, the setting of the setting section 26 is cancelled, and the control circuit 11 sends out pseudo incoming signals to all of the extensions a to f to send the facsimile to the facsimile. The transfer function does not work. That is, the extension f can be used as the extension to which the telephone C is connected, like the other extensions a to e. In the above case, the operation was performed at home, but the answering switch P is provided in consideration of the incoming call to the facsimile when going out. The operation state of the answering switch P is detected by the switch operation detection circuit 27. That is, the switch operation detection circuit 27 applies a voltage to the answering switch P, and detects the operating state from a voltage change when the answering switch P is operated. When the operation state of the answering switch P is detected, a detection output is sent to the control circuit 11.

When an outside line is received while the answering switch P is set to the outside state, the control circuit 11
Is a pseudo incoming signal generating circuit 2 for all extensions a to f
From 0, a pseudo incoming signal is sent out, which causes the facsimile to automatically respond. By the way, NTT provides a non-ringing incoming call service in a facsimile communication network, and some users desire to receive this service. Therefore, in the case of the present embodiment, when the above-mentioned service is received, 1 is transmitted as a facsimile calling signal.
A call signal detection circuit 28 for detecting a 300 Hz facsimile call signal is provided. In other words, when a facsimile calling signal is detected, a pseudo incoming signal is sent from the pseudo incoming signal detection circuit 20 only to the extension f to which the facsimile is connected, so that the user does not perform a transfer operation when at home, The facsimile can be automatically operated.

Further, the home telephone exchange system includes a communication device U having the same communication function as the door phone D, and is used for communication between rooms. That is, the communication device U is installed in, for example, a washroom, a toilet, or a garage, and a monitoring control circuit 31 and a communication circuit 32 are provided for the communication device U. It does.

Here, in the case of the present embodiment, simultaneous broadcasting can be performed from the communication device U and the telephone outlet B, and whether or not to select this simultaneous broadcasting function is set by the setting unit 26. Now, when the broadcast function is selected, for 30 seconds after the ringing tone is sounded from the communication device U and the telephone outlet B, for example, even if the handset of the telephone C is not picked up, the telephone outlet A communication call can be made from B and the communication device U. However, in this case, if the receiver of any telephone C is picked up, the extensions corresponding to the telephone C and the telephone C on the calling side are connected to each other, and the telephone is in an extension communication state.

If the broadcast function is not selected, the broadcast cannot be performed. However, in this case, when a dial signal corresponding to the broadcast is input, a general call for calling each extension is performed. This function can be used as a function, and when any of the telephones C answers, an internal telephone call state is set. As described above, in the home telephone exchange system in which the outside line call, the inside line call, the door phone call, and the indoor communication can be performed, there is a case where each call state conflicts. Therefore, in this home telephone exchange system, the priorities are set in the order of external call, intercom call, indoor call, and internal call. In addition,
Door phone calls and indoor communications are given priority over later calls. Assuming that the control circuit 11 is multifunctional as described above,
In this case, the telephone exchange A shown in FIG. 2 uses two input / output port expansion units 30.

By the way, recently, a commercially available telephone C
Cordless phones have become widespread. Therefore, it is conceivable to use this cordless phone in the home telephone exchange system. However, if a cordless phone is used as the telephone C in the above-mentioned home telephone exchange system, a quick incoming call function of the cordless phone (a function that enables a call by simply lifting the cordless portion of the cordless phone) cannot be used for an extension call or a door phone call. . In other words, the quick call function does not function unless an incoming signal is received, so if the cordless part is lifted and an alarm sounds after a few seconds elapse, if the external call or internal call button is not pressed, the internal call The state is cut off, and in the case of a cordless phone, the normal telephone C
Similarly to the above, there is a problem that an extension call cannot be made without any operation.

Therefore, in this embodiment, the setting section 26 sets whether or not a cordless telephone is connected as the telephone C. When the cordless phone is used, after delivery of the extension call or intercom call using telephone outlet B for each extension ( "ping-pong" or "Puppuppu over" sound), the pseudo ringing tone generator for each extension L ₁ 20 sends a pseudo incoming signal.
However, the transmission time is set to 30 seconds. In this way, even when a cordless phone is used as the telephone C, extension calls and door phone calls can be performed without lifting the cordless portion and performing any operation, that is, using the quick incoming call function of the cordless phone as it is. . However, when a facsimile is connected to the extension f, it is necessary not to send the pseudo incoming signal to the extension f to which the facsimile is connected so that the facsimile does not automatically respond with the pseudo incoming signal. There is. When all the telephones C are not cordless phones, no pseudo incoming signal is transmitted.

When both a cordless telephone and a normal telephone are used as the telephone C, the built-in bell rings unnecessarily on the normal telephone C, and the call is duplicated. Therefore, if the extension unit connected to the cordless phone is specified and set by, for example, a dip switch, etc., the setting unit 26 can be used to send a pseudo incoming signal only to the extension unit connected to the cordless phone. This is preferable because it is possible to prevent the built-in bell of the telephone C from ringing unnecessarily after a call or a door phone call.

The above is an overall description of the home telephone exchange system. The features of the present embodiment will be described below. The make-time of the off-hook and DP signals of the telephone C, DC閉結circuit becomes a DC閉結state provided in the phone C, while the extension L ₁ is at a low impedance (50
300Ω). Therefore, at this time, the current supplied from the driving power supply to the communication current supply circuit 4 increases. Therefore,
In this embodiment, if detects the current change, rather than an extension L ₁ side which is connected directly to the external line L _0, the no speech current supply circuit 4 side be connected to the external line L _0, the hook state and It focuses on the fact that a DP signal can be detected.

In this embodiment, as shown in FIG. 1, the resistance R ₂ inserted in the path for supplying the driving power to the communication current supply circuit 4 and the current supplied to the communication current supply circuit 4 increase. a transistor Q ₁ which is turned on when the voltage drop across the resistor R ₂ is increased, there is constituted a hook · DP detection circuit 2 by the resistors R _3, R ₄ which are connected in series to the transistor Q ₁ .

[0036] Now, when the phone C is on-hook, and during break DP signal in a state where the DC閉結circuit does not direct閉結phone C, since the voltage across the extension L ₁ is a high impedance, speech current Supply circuit 4
Does not supply talk current. Thus, the transistor Q ₁ is turned off, the voltage input to the control circuit 11 is 0
V.

[0037] Conversely, when the phone C is off-hook, and at the time of makeup DP signal, DC閉結circuit of the telephone C is DC閉結, since the voltage across the extension L ₁ becomes low-impedance, speech current A communication current is supplied from the supply circuit 4 . Thus, the transistor Q ₁ is turned on, the voltage input to the control circuit 11 is a voltage obtained by dividing the voltage obtained by subtracting the collector-emitter voltage when the on transistor Q ₁ from the drive power source by resistor R _3, R ₄ Becomes In this embodiment, since the control circuit 11 is constituted by the CPU, the divided voltage by the resistors R ₃ and R ₄ is about 5
V.

[0038] Since the hook · DP detection circuit 2 detects a hook state and the DP signal of the telephone C in the path that is not connected directly to the external line L _1, the external line L ₀ and the hook · DP detected between the telephone set C As in the case where the circuit 2 is provided, the call voice is not attenuated. Moreover, in this way to insert the external line L ₁ directly unconnected route hook · DP detection circuit 2, it is not necessary to use a photocoupler PC, yet does not require a like circuit for absorbing lightning surges, the circuit it is possible to configure easily, it is possible to reduce the cost, if the extension L ₁ connecting the phone C as in this embodiment is six lines can be expected to drastically reduce cost.

By the way, as described above, during an outside line call, the hook is placed at a position where hooking from the telephone C cannot be detected.
When the DP detection circuit 2 is inserted, it is impossible to detect a hooking operation when holding is performed. Therefore, if the control circuit 11 detects the hooking operation from the output of the polarity inversion detection circuit 16, the holding operation can be performed in the same manner as in the related art.

[0040]

As described above, according to the present invention, a call current supply circuit for supplying a call current to a telephone in a state where an extension is disconnected from an outside line, and a current supplied from a power supply to the call current supply circuit is detected. Since a hook / DP detection circuit for detecting a hook state of a telephone and a dial pulse (DP) signal is provided, the hook / DP detection circuit can be provided on a path which is not directly connected to an external line. If a hook / DP detection circuit is inserted in a path that is not directly connected to an outside line without attenuating call voice as in the case where a hook / DP detection circuit is provided between them, it is necessary to use a photocoupler. Further, there is an advantage that the circuit configuration can be simplified and the cost can be reduced without requiring a circuit for absorbing the induced lightning surge.

[Brief description of the drawings]

FIG. 1 is a specific circuit diagram showing a configuration of a main part of an embodiment of the present invention.

FIG. 2 is a block diagram showing a circuit configuration of a telephone exchange provided with the same.

FIG. 3 is an explanatory diagram showing a system configuration of the above.

FIG. 4 is a specific circuit diagram showing a configuration of a main part of a conventional example.

[Explanation of symbols]

A telephone exchange C phone L ₀ outside line L ₁ extension 2 hooks · DP detection circuit 4 speech current supply circuit Q ₁ transistor R ₂ to R ₄ the resistance

──────────────────────────────────────────────────続 き Continued on the front page (58) Field surveyed (Int. Cl. ⁷ , DB name) H04Q 3/58 101-107 H04M 19/08 H04Q 1/32 H04Q 3/72

Claims (2)

(57) [Claims] 1. A telephone exchange connected to an external line, which is a public telephone line, and a plurality of commercially available telephones connected to the telephone exchange via an extension, wherein the telephone exchange has an external line and an extension or both extensions. In a home telephone exchange system for switching between communication paths to allow external and internal calls with the telephone, a telephone current supply circuit for supplying a telephone current to the telephone while the internal line is disconnected from the external line; and The current supplied from the power supply to the call current supply circuit is detected to detect the hook state of the telephone and the dial pulse (D
P) A home telephone exchange system comprising a hook / DP detection circuit for detecting a signal. 2. The hook / DP detection circuit is applied via a detection resistor for detecting a current supplied from a power supply to a call current supply circuit, a transistor for turning on / off by a voltage drop of the detection resistor, and a transistor. And a voltage dividing resistor for dividing a voltage.
The home telephone exchange system as described.