Why does MTN allow numbers to port that belong to other people.

This is not a very good year. How is it that MTN can just allow someone to port a number that already belongs to someone. I have been battling for 2 days to try and get my number back and no one seems to be able to help me!!! This is pathetic. This is the 3rd time since the beginning of last year!!!! I have had to go and get a new sim card under instruction from MTN and they STILL CAN NOT HELP ME!!!!! I WANT MY NUMBER BACK AS I USE IT FOR BUSINESS AND I AM LOOSING CLIENTS!!!!!