In message , Kroma
writes

Is there a cordless DECT or similar phone plugged in upstairs? Therefore
you are cutting the power to its base unit. Surely you don't mean that a
plugged in normal phone stops working when you turn the power off do you?

Yes I do - I may be daft but not THAT daft. I was shocked too!! The only
explanation I could think of was that it goes through the alarm somehow
(it's only the upstairs extension which goes off - all the sockets
downstairs continue to work as normal).

Daz



Daz,
I feel it might be time to very carefully trace the cabling and
see just where it goes and what else it connects to. The phone wiring
should not be connected to anything else and certainly not anything
connected to a ring main! As was suggested maybe a phone line extender,
although I thought those only regenerated ringing volts and would have
left 2 and 5 alone. Maybe not though.
Hopefully there is a simple explanation to it all and there is something
legitimate missing from your description but I'm not sure what at the
moment. Maybe the previous suggestion of "call an exorcist" would be
worth while?

Best of luck and nice knowing you.
--
Bill