Bad Service

I have been getting a lot of calls from MBD. They said I have an account with MTN which is outstanding or still owing. I had the account 8 years ago. They said I owe R2600 and initial amount which I owe MTN is R900. I was paying the amount until I went to MTN branch in centurion mall and I was told my account does not exist anymore and it has been allocated to someone else and its not even in their legal department as it was written off. MBD bought a book at MTN which they collect and money is for them. According to MTN records I owe them nothing. According to MBD the account was handed over in 2009 and they only call me this year. I ask them a proof from MTN they can't give me. They said maximum term to keep account is a year and they kept mine for 6 years. They just wanted to milk me money. I would not pay it until I get a proof that I owe MTN money and they send me a letter with the proof.