GB1180140A
(en )
1970-02-04
Automatic Signal Transmission System
US3865995A
(en )
1975-02-11
Universal subscriber{3 s line circuit in a key telephone system
GB507132A
(enrdf_load_stackoverflow )
US4214127A
(en )
1980-07-22
Intrinsically safe telephone network
GB388752A
(en )
1933-02-28
Improvements in or relating to telephone systems
US2024581A
(en )
1935-12-17
Telegraph toll system
US2200820A
(en )
1940-05-14
Automatic telephone system
US1895112A
(en )
1933-01-24
Printing telegraph exchange system
US2410866A
(en )
1946-11-12
Telephone system
US2241258A
(en )
1941-05-06
Teletypewriter switching system
US1996516A
(en )
1935-04-02
Printing telegraph private branch exchange system
US2822423A
(en )
1958-02-04
Fire and police intercommunication system
US2143000A
(en )
1939-01-10
Telegraph system
US2173551A
(en )
1939-09-19
Communication system
US2424244A
(en )
1947-07-22
Circuit for locking service observing teletypewriter during supervisory signal reception
US1592646A
(en )
1926-07-13
Automatic telephone system
US2342586A
(en )
1944-02-22
Repeater system
US1895097A
(en )
1933-01-24
Printing telegraph exchange switching system
GB527336A
(en )
1940-10-07
Printing telegraph system
US3571531A
(en )
1971-03-16
Selective automatic service observing in communication systems
US2093084A
(en )
1937-09-14
System for giving teletypewriter service over telephone lines
US1931603A
(en )
1933-10-24
Telephone system
US1632051A
(en )
1927-06-14
Telephone system
US2923763A
(en )
1960-02-02
Printing telegraph system
US3997735A
(en )
1976-12-14
Telephone station make-busy circuit