[a good help]

likely cause is a faulty extension wire, go to the master socket and remove the face plate, then call the line listen and hang up then see if the continious ringing is still there? as others said above have you also tried another handset, if you do those and you still have the issue then i would call customer services, but do remove the extensions to ensure that you prove the fault is external, if you have say ex BT extension sockets that are now connected to the NTL line and a engineer comes to the house and finds that they are the cause of the fault, then you could be liable to a £25 service charge.