everything is normal until we get an incoming call, then we lose the broadband for two minutes. The rest of the time we get a dial tone on the phone as usual. Afraid I have no idea what the test socket is.
If you remove the lower faceplate portion of the master socket you will reveal a test socket. This is the demarcation point between your responsibility and Openreach's . You need to test from there to prove the fault is outside of your wiring.
Do you still use a filter for your broadband? If so have you tried replacing it? If you have extension socket do you filters in them if in use?
As you are not doing the checks then just phone 151 and report a phone fault and get engineer visit. Bear in mind if problem is internal then you will be billed for engineer visit £85