Do any of your sockets look like this?
http://www.broadband-finder.co.uk/bl...r_socket_2.jpg
If one does then you have a test socket. If you have more than one socket on the line you should have one of these, if not there is little you can do to troubleshoot this issue, you can call BT and go through steps with them but they will not send out an engineer unless you agree to the risk of call out fees so don't worry about a suprise charge.
If you have a master / test socket you need to isolate it for testing, if you cannot do that yourself you will need to get a phone engineer in or BT in to do it for you to sort out the problem.