Contracts ***** and theft

I have a contract with mtn and someone has access to my contract number and has been making transactions such as transferring airtime, pay for me calls, buying data bundle using advances where my bills move from R980 to R2900 for the past 3 months and when i call them they blame me when I report and said this transaction were made manually. When I switch of my fone and call my number using another it rings to show that I'm sharing a my contract number with someone and they deny it. So they lodge an investigation and never come back to until date