ADSL2 working, phone line out

[abthomp]

This is probably a dumb question but we currently have an outage in our area due to some numpty digging up cables.

Our landline phone isn't working for either incoming or outgoing calls but our ADSL2+ is going great guns

The matter is reported and being attended to but I was just curious as to how this works

[cf4]

There could be a fault in the exchange switching equipment or there could be a high resistance DC (Direct Current Fault) fault on the cable. With a high resistance DC cable fault the cable is able to transmit high frequency AC (tone) but not direct current.

For the phone to work the line needs to be able to pass DC current so that the line can be looped when picking up the handset to initiate a call and also send a signal to the exchange when you pick up the handset on when you receive a call. If the phone uses pulse dialing (rotary dial) it also needs the line to pass DC current. Most phones are also powered by the DC current from the exchange.

The cable only needs to be able to pass high frequency AC (tone) for ADSL to work.

A short circuit to earth on one of the wires of the cable pair might  prevent phone working but might only reduce the performance of the ADSL connection.

[abthomp]

There you go - I've learnt something today. Wasn't a totally waster day after all. Thanks