Leyburn Derek, Davey Murray Allan: Telephone trunk testing system. The Bell Telephone Company of Canada, December 5, 1972: US3705275 (5 worldwide citation)

A system for testing telephone trunks, particularly trunks in tandem extending between three or more crossbar offices, in which each trunk circuit is given a line location and a local telephone number in its own office. The switching equipment in each office is arranged so that when it receives a tr ...