You'll find that its most probably caused by either the wiring in the phone or if you have one a double adapter. There is nothing at all wrong with binatone phones. They are a brand that fully support cable telecom lines and their caller display units are used by many techs for testing. Phillips cordless are the ones to shun as I find that batterys don't have much life in them.
Before consigning your phones to the bin or changing them try a little play about. 1st remove all phones from the line and try your phones in different combinations starting from the master socket to see in what combination the fault occurs. But be careful about leaving a iffy phone set-up thinking it'll get better. You may end up with ring-trip where the phone/ext/adapter actully opens the line. In most cases the switch put your line to plo [lock out] which prevents incoming calls and outgoing calls but sometimes the phone can end up dialing 123,100,112 and either costing you money or causing un needed call outs for the emergency services. They can contact ntl and ask them to temp terminate your line if its a repeated issue.
Let us know how you get on
