I thought from your initial post that you had spoken to them, but your last post suggested it was a message left on the answerphone. I wouldn't have sent them an email as they now have another method of pestering you. These people tend not to give up in a hurry and always assume that you are making up stories just to avoid paying the debt. Whatever you do, don't give them your address and make sure you are listed ex-directory and I would register with the Telephone Preference Service as well.
Next time they contact you, if they aren't apologising and if they are still pestering, or demanding information about the person that owes the debt, tell them quite plainly that it is a new telephone number to you, you don't know the person that they want and not to call your number as it is now, again, or to email you demanding money or information.
If necessary remind them of what you just told them and that should they contact you again, it will amount to Harassment - a criminal offence, and you will report them to the Police.
Good luck.
