Our household phones (about 6 of them) sometimes...

I'm a BT engineer and it sounds like you're overloading the circuit, try uplugging a few phones and see how it goes. failing that go to your main socket (you will notice the screws on the front are slightly lower than central and it looks like it has 2 sections) unscrew the front panel and remove the lower half, you should see a phone socket on the right hand side, now plug a phone into it (this is your test socket, it is the direct line from the exchange) if your problem still exsists with just the one phone in the test socket call 150 and report a fault. feel free to e-mail me scottsavill@btinternet.com if you still have probs