This is called a jumpering fault. As your service came across fine it'll likely be the engineer who installed the talk talk service for someone else who got the cables mixed up-probably at the cabinet. You have the other persons service and they have yours. The broadband won't work because you're router is trying to log to pnet down a talk talk line.
Either side can pick it up-you'll need a phone engineer. Probably easier to get from your side though-if the other customer is a new customer they may not be aware yet, plus however much people slate pnet for the phone times at least you'll be able to just get straight to the point with it. Last person I spoke to on talk talk with a problem lost their mind with the language barrier and unnecessary testing (They had them doing test socket tests for a visible busted router!) Plus best to make pnet aware anyway as any calls the other person makes would go on your bill.
