You've started where I would, by checking that the problem doesn't lie with the phone, but have you checked whether it could be caused by something else connected to your phone line?
Before calling TalkTalk, try unplugging everything connected to your phone line (including on any extensions) such as an answering machine, plug-in extension leads, and (importantly) your ADSL filter and router. Have only your phone plugged in (using the main socket, not an extension) and see if the problem persists. If it does, you can then confidently tell TalkTalk that there's a line fault.
Chris