They are accusing me of having a contract with them on a total different cell no than mine

They are accusing me of having a contract with them on a total different cell no than mine. I've never in my life had a cellphone contract and I've had the same number for many years. From last year they say I have a contract, telephonic conversation. I went to MTN shop and they confirmed that I don't have a contract but it was a telesales that contacted me. I remember that phone call. I did not take the contract since I've been pay as you go for all these years with the same nr for many years. CSS keeps harassing me about payment and now I'm threatened with default. No debit orders go off my bank and I receive no account for this so called contract. I even made an affidavit with SAPS to confirm this and emailed it to them. But they don't stop. I'm credit listed for nothing. I'm convinced it's a ****.