Centrally controlled branch telephone exchange - allows enquiring station to hold addressed station whilst calling up third station

摘要

The branch telephone exchange has the loop circuit that already exists for an enquiring station preserved when the dial switch is operated at the enquiring station, in order to select a telephone service (e.g. a trhee-way connection). The already existing loop circuit for the enquired (called back) station is disconnected by special switching by switches arranged in a given way and another loop circuit constructed that contains no winding belonging to the line transformer. A tone can be applied to both stations and may have a different frequency for each station. If selection of the wanted service is not complete after a given time, then the enquiry state existing before the dial switch was operated is returned to.