Limit based line changed without my authority

I have a contract with MTN which was under Nashua Mobile before and it had a limit of R50.00 incase I finish my free minutes,data and smses and with the history of my account it has never been over but I recently received a statement from MTN that my account is R1742.73 overdue which is something I don't understand on how did it happen because I've never gave a permission to anyone to make my number an open line.My services have been suspended due to this problem as they also tried debiting from the wrong account which I also gave them new banking details but they keep going to the wrong one.It's not my responsibility that MTN opened the line because I set the limit on the account so that I don't overspend but now I'm being punished for their own faults.<br> <br> I have experienced bad service with MTN for the third time with MY contracts within a period of 1year which resulted in me cancelling my other contract.Why do things have to change when MTN took over from Nashua Mobile?it's like MOVING from GOOD SERVICE TO THE POOREST SERVICE.I NOW DON'T TRUST MTN AS A SERVICE PROVIDER BECAUSE IT LOOKS LIKE IT'S A **** OPERATING SERVICE PROVIDER REGARDING MY LINE BEEN OPEN WITH NO LIMITS.