US phones (and most phones worldwide) use the middle pair of wires (3+4) of an RJ-11 modem/telephone cable to carry the voice and ring signal. Most UK powered phones including DECT phones also use the middle pair of wires. However, BT/UK master sockets include a ring capacitor that provides a ring circuit via a third wire (wire 3 on the BT plug end), which is required by some telphone handsets. This adapter has a built in ring capacitor to provide the ring signal via the third wire when connecting a phone to an FXS port on a PCI card or Gateway/ATA.
