Are you using any phone extensions ??
I had a similar problem a while ago and got a VM engineer out. He located where the line came into the house and then disconnected the lead coming out of there, to the rest of the extensions. Everything worked fine with the extensions disconected, so now the rest of my phones in the house are cordless.
If you have any extensions, disconnect them and plug your phone into the main socket. It's not enough to unplug the phones from each socket, you've got to disconnect the extension that comes out from the main box, so that the only phone cable that is "live", is the one coming into your house into the main box - no extensions should be "live".
If you don't have any extensions, then just ignore all of my waffle lol :P