generally, the people who are unhappy with their services are people who owe money - they seem to feel it disgusting that a company is trying to trace them to honour their commitments.
As stated, if you do not owe money, then you have absolutely nothing to fear - you obviously cannot be responsible for someone else's debt.
If you moved in 3 years ago and took on a BT phone number, this number may have been someone else's number previously who does owe them money. It could also be a mistake when someone transferred the phone number into a computer system - these things happen frequently.
All you are doing by not telling them that this is nothing to do with you, is wasting their time and money - you may feel happy about that - and of course it is your call.
But, please try and remember, all these consumer sites that tell people how to stop paying charges etc with banks have an important implication - it means that banks are not making so much money - and that is something they will not be happy with.
Currently in the UK, we have totally free personal banking - bear in mind that with businesses (and most personal accounts in other countries) you get charged for each cheque you write, each time you withdraw money etc. - this is something I can foresee happening here soon - all due to these consumer websites who are trying to hide the fact that a person made a contract with a bank and is breaking that contract.
rant over ;-)