What you want to do is go and plug a phone into the socket and see if you get a dialing tone. if not take the front socket off and there should be two wires connected to i think its 2 and 5 (google it) it thats connected its at the exchange and as i work on cellular equipment havent really worked on one but would have a bash! what you find is most of the time BT realise the phone hasnt been used and turn it off but depends how long the house has been on the market. whatever happens unfortuately they'll get the money out of you some where along the line!