Loyal Customer Been Taken For Ride

I went in store to do a sim swop after I lost my phone. I was told that I am blacklisted and i cannot be assisted. I was told i need to pay a certain amount and i will be assisted. I was retrenched and i did let Telkom know and when i got the job i did the same again and started paying. What is worse is that my number is prepaid, so if this person that has my phone can do ***** with a cellphone number that is registered in my name what happens next. Please explain to me why i am blacklisted even though I am paying you monthly.