Have the same problem. It's a old BT line but I'm with SKY. The problem is we have filters fitted as well and it still happens. Checked the wiring and all seams fine. I'm an electrical engineer as well and it's baffling me
This is the sort of problem that has probably been caused by something changing if it was working OK previously. It sounds mostly like an ADSL filter missing or faulty somewhere.
Are any of the 'phones or the fax machine new?
Is the new main socket on the same line as the one that used to work OK?
Are you sure that you have ADSL filters plugged into EVERY extension on the line which is causing the problem?
Did you change or plug anything different in when the problem started?
It might be an idea to log into the router on 192.168.0.1, username Admin and password Sky, then bring up the Show Statistics page in the Router Status menu. Keep an eye on the Noise Margin figure keep watching that screen while you pick up a phone and see if it changes.
Finally, check if there is a wire, usually orange/white, connected to terminal 3 of the master socket. If so, disconnect it and see if that makes any difference as you do not need it.