Prepaid no that owes R10 000.00

I had a cell phone contract with Telkom and in 2017 and ended my contract. I was told to pay R5000.00 + of which I did. I then app**** for a prepaid no. Last year November I was again harassed and they claimed that I still owed them over R3000.00 + . I requested that they send me a • (Anthipi)paid up letter, after back and forth with waiting for my letter there was not response from their side. Now I am again getting sms and they claim I owe them R10 000.00 how is this possible while I have a prepaid cell no. When I visit a Telkom store no one is able to help.