MTN line gets cancelled and still get billed

Nashua Line was cancelled in October, got a call back to say they can't cancel as they have sold the base to MTN.Not my problem as i cancelled and did all the correct things. To date MTN is billing me and i cannot for the life of me understand why i cancelled my line which I'm entitled to and they ignore and keep billing. I have been trying for three months to cancel and MTN is impossible in replying to mails and calls. Please can some one advise me or help me! Customer care at MTN is ridiculous and no one gets back to you and no one there knows what to do.