US4196317A
(en )
1980-04-01
Telephone intercom system
US6633589B1
(en )
2003-10-14
Switching apparatus for use in telephone house wiring
US4088846A
(en )
1978-05-09
Hold-plus-intercom for single-line stations
US5966432A
(en )
1999-10-12
Remote answering of doorbell
CA2101080A1
(en )
1992-07-30
Method for signalling/communication within an entrance telephone system and an entrance telephone system communicating according to the method
US4536615A
(en )
1985-08-20
Key telephone equipment and associated method
US4572928A
(en )
1986-02-25
Key telephone system
US2801287A
(en )
1957-07-30
Control of regular and distant talking subscribers sets
US3557318A
(en )
1971-01-19
Apartment house telephone system
US3865995A
(en )
1975-02-11
Universal subscriber{3 s line circuit in a key telephone system
US3965308A
(en )
1976-06-22
Line card circuit
US3803360A
(en )
1974-04-09
Switching apparatus for connecting a data terminal to a plurality of communications networks
GB305730A
(enExample )
US2771511A
(en )
1956-11-20
Line clearing device for receiver-off-hook condition
US2200820A
(en )
1940-05-14
Automatic telephone system
US4283603A
(en )
1981-08-11
On-hook/off-hook status check circuit
US2813152A
(en )
1957-11-12
Telephone intercommunication system
US2065642A
(en )
1936-12-29
Combined telephone and fire dispatch system
US4868847A
(en )
1989-09-19
Telephone intercom device
EP0235883A1
(en )
1987-09-09
Telephone intercom device
US3133152A
(en )
1964-05-12
Teletypewriter switchboard
US4130737A
(en )
1978-12-19
Two-link intercom system
US3055985A
(en )
1962-09-25
Residential or farm intercom telephone system
JPS59178851A
(ja )
1984-10-11
電話機回路
US1938709A
(en )
1933-12-12
Telephone system