Priority date (The priority date is an assumption and is not a legal conclusion. Google has not performed a legal analysis and makes no representation as to the accuracy of the date listed.)
Publication of NL3569CpublicationCriticalpatent/NL3569C/xx
H04M3/60—Semi-automatic systems, i.e. in which the numerical selection of the outgoing line is under the control of an operator
H04M3/62—Keyboard equipment
H—ELECTRICITY
H04—ELECTRIC COMMUNICATION TECHNIQUE
H04M—TELEPHONIC COMMUNICATION
H04M3/00—Automatic or semi-automatic exchanges
H04M3/08—Indicating faults in circuits or apparatus
H04M3/12—Marking faulty circuits "busy"; Enabling equipment to disengage itself from faulty circuits ; Using redundant circuits; Response of a circuit, apparatus or system to an error