***** on fixed contract

My mother has a fixed monthly contract with MTN. For the past few months someone accessed her “credit” with mtn, and have purchased airtime of several thousand rands. We went to the branch at Hillcrest and Brooklyn several times to report this, with no luck. They simply refuse to give us a reference number, which means the ***** on my mother’s account just continues. I am at my wits end! Is this an inside job?