ISPs often use that: 'If we don't find anything wrong on our side we will give you a huge bill' in order to deter people from asking for an engineer visit. This is because the engineering is supplied by Openreach (BT) and the ISP has to pay for the visit.
If you still have the problem using the test socket then it is definitely an external fault and you need to tell your ISP that and get Openreach on the case. (You can't call Openreach personally as they only work for ISPs.)
t
June 2008