Thanks folks,
BT have admitted that it was their responsibility, and have promised to remove the charge from my account. We shall see, but worth persisting. Thanks for your thoughts....
the line box is the nte 5a box fitted by bt. the system was tested w/o any equipment connected and with working equipment connected to the test socket. the problem was a faulty box (short circuit) probably caused by failure of the zener diode surge protection to produce a short circuit (loop)
hope someone can advise. following a recent storm, telephone went dead. called BT who asked me to carry out self test on line box. This showed that it was not my equipment at fault and engineer was sent out on the basis of this who traced a loop fault in the faceplate of the linebox, replaced...