The BT431A is the UK telephone connector for telephone and ADSL connections. Pins 2 and 5 carry the primary phone line. Not compatible with RJ11 without an adapter.
| Pin | Name | Function | Type |
|---|---|---|---|
| 1 | Spare | Unused | I/O |
| 2 | A Wire | Phone line A tip | BUS |
| 3 | Spare | Unused | I/O |
| 4 | Spare | Unused | I/O |
| 5 | B Wire | Phone line B ring | BUS |
| 6 | Spare | Unused | I/O |