Signaling is simplified by the adoption of a universal numbering plan. Addressing between centers and from subscriber keysets uses the same inband v-f combinations. Supervision on trunks is accomplished with single frequency tones. For subscribers on local metallic lines advantage is taken of the 4-wire phantom for dc loop supervision and battery feed to normal subscriber instruments. Protection against fading or interruption of the trunk transmission path is achieved by a unique interlocking of all addressing and supervisory signals. Simulation is minimized by using a special grouping of v-f combinations and timed single frequencies.