Abstract: The disclosed communication system operating mode signaling method and arrangement utilizes dc current signaling over the loop facility connecting a customer to a central office (CO). The dc current on the facility provides concurrent on-hook/off-hook signaling and voice/digital data operating mode signaling between the customer and CO. A current flow in a first direction over the facility signals a voice operating mode while a current flow in a second direction signals a digital data operating mode. A customer signals a change in the operating mode by reversing the direction of allowed current flow at the customer unit. The resulting interruption of the current flow is detected at the CO which responds by reversing the direction of current flow over the facility. The reappearance of current flow over the facility distinguishes a customer's change in operating mode from an on-hook condition.