BT (pre-431A) Phone Jack

Before BT 431A was standardised in 1981, the British telephone system used a family of round-pin connectors codified as BS 415 (with variants 1A, 1B, and 1C). These older plugs are bulky, have separate ringer/line/earth pins, and remain in use in some remote former-British-Commonwealth installations where infrastructure has never been modernised.

Countries still listing the older "BT" jack as their primary standard include the Falkland Islands, the Gambia, and Gibraltar. In these locations a BT-to-RJ-11 adapter (the same one used for BT 431A in the UK) usually works, since modern BT adapters are backward-compatible with older sockets.