[Lord Nikon]

Since BT aren't interested run the following test at your parent's place.
1) Unplug all phones from the master socket
2) Remove the faceplate, if you're on the master socket you should have a screw either side on the lower half of the faceplate, remove them then pull the faceplate forwards, there should be another socket inside. Try calling the line then, if it does the same then go to step 3, if not then it's internal extension wiring that's at fault.
3) Remove the other 2 screws and check that there is only 1 wire going to the A and 1 to the B terminals in the faceplate. If not then someone has wired the extensions incorrectly
4) If there is only 1 wire to A and 1 to B, Call BT, INSIST on an engineer visit, they'll tell you if they can't find a fault or if it's a fault with your internal extension wiring they'll charge you. Doesn't matter - you've eliminated the extension wiring with the above tests, from there on in it's ENTIRELY their responsibility, there's a fault you can repeatedly demonstrate so it's their problem.