MTN unauthorized deduction

Hi, My MTN subscription is 225 been paying that no problem, today they deducted another 75 without notification and i did not approve the debit order. It is not the first time i've had this issuewith MTN.I don't mind paying if i know what i'm paying for.I also know that they won't even bother to call me. Bare in mind i have been a subscriber since 1998 and this contract for started 2004.

Regards Alwon