The central office (CO) sends a signal to the originating switch when the far end answers a call. If a call travels over a private network before the call reaches the CO, the signal is transmitted back over the private network to the originating switch. This method is extremely accurate, but network answer supervision is unavailable over most loop-start trunks. For example, network answer supervision is unavailable over CO, foreign exchange (FX), and Wide Area Telecommunications Service (WATS) trunks in the US.