As pair gain electronics and point-to-point microwave radio links came into widespread use throughout the public switched telephone network, telephone companies filed tariffs to eliminate their past responsibility of providing leased circuits with direct current continuity. If the base station were located across town in an area served by a different telephone exchange, the only available circuits reaching the distant exchange might be a single voice-grade channel in a D-4 channel bank on a DS-1 or a single microwave radio baseband channel. Tone remotes became necessary with the wide use of telephone carrier or multiplexing equipment. They require only a voice-grade audio path with roughly flat equalization from 300-3,000 Hz. A circuit that could pass audio in both directions could be used for remote control.