That is for use on the old BT wiring plans which were 2-wire with no
sockets. They need rewiring to work on a modern system which converts
them to 3-wire ringing to avoid bell tinkle. Its almost certainly been
converted to 3-wire possibly as per, but I don't know if there are
GPO Telephone No. 330, 332 & 333
Telephone is wired as follows. The standard diagrams are N430, N432 & N433.
1. Remove strap T10 - T11.
2. Remove strap T11 - T12.
3. Insert strap T8 - T9.
4. Insert strap T1 - T2.
5. Insert a 3.3K ohm resistor between T11 and T12.
6. Rectifier No. 205 (if needed) inserted between T6 & T5.
7. Red wire of line cord to T9.
8. White wire of line cord to T1.
9. Blue wire of line cord to T11.
10. Green wire of line cord to T3.
> German telephones also use these 2 wires for ringing the bell.
> Although, some older ones have an additional contact for an additional
> bell that is being disconnected when the phone is hooked off.
> I can confirm that the RJ11 of the German 7490 supports the bell and
> the normal phone line on 2 wires.