unfair treatment by telkom mobile

I had an account with telkom which i closed last year but did not continue using the number, after sometime I went back so I can make the number prepaid. I was told that I owe R1004 which I was surprised but I paid the amount after what they explained to me even though I was not completly happy about it, after paying I changed my number to prepaid but after that I am getting messages from some debt collectors saying I am owing telkom more than R1600 which I must pay immidiatly. I am confused with this whole situation since I have paid my account in full.