Billed for someone elses number

In August MTN assigned a number to my contract that is not mine. I queried it and the referred me to an iStore that linked this number to me. I phoned the iStore and they said they will sort out their mistake as I never apl**** for this contract. Now I get a bill from MTN for R7000 for the cancellation of the contract that I did not apply for. I phoned 808, they said it comes from the retention department, and after holding the line for 30 min I was informed I must phoned customer care. They said they only do billing and I must proof that I didn't sign up for that contract. Why can MTN bill without a contract in place?