MTN issued a SIM with my mobile number to another person

As a Vodacom contract customer for 20 years after porting from MTN, MTN confirmed that they have also issued my mobile number on a pre-paid contract to another person. In my view this cannot be legal and opens my mobile number to *****. I have reported this through their (very bad) call centre and also at a MTN retail shop. It has not being corrected. Can someone at MTN please help?