Differentiation system between PBX trunk calls to different zones - is based on different return signals from public exchange with local and remote calls using dissimilar pulse times

摘要

Return signals from the public exchange marking both trunk (FG) and local (NG) calls, are charge pulses modulated with an ac voltage. Pulse train (P1) for local calls differs from the pulse train (P2) for trunk calls in at least one respect. An evaluating device (BE) at the PBX evaluates the difference between the return signals, and marks the PBX extension connected to a distant subscriber. In a typical arrangement the pulse train for the local calls (I1) is longer than that of the remote calls (I2).