BT 431A Phone Jack
Also known as the British Telecom 431A.
About the BT 431A Standard
BT 431A is the standard British telephone plug, introduced in 1981 to replace the older BS 415 round-pin connectors. It is a six-position plug with a key-shaped retention tab that distinguishes it physically from RJ-11 — the two plugs are visually similar but not interchangeable, and forcing one into the other socket damages both.
UK premises wiring runs from a master socket (NTE5 demarcation block) through BT 431A extension sockets. The master socket itself has a hidden RJ-11-compatible test interface behind the removable faceplate, but the user-facing outlets are BT 431A. Modern broadband filters (ADSL splitters) also use BT 431A on the line side.
For dial-up modem, fax, or analogue phone use in the UK, travelers from North America need a BT-to-RJ-11 adapter. The BT 431A connector is also used in Ireland, Cyprus, Malta, Gibraltar, and historically across Commonwealth installations.
Adapters for BT 431A
Countries Using BT 431A
1 country uses BT 431A. Click any country for its full connectivity, voltage, and adapter guide.