Unhappy client from MTN

I have a contract with MTN, 399 was the agreed amount, but last month they debited 787.00, worse this month the statement says 943.00 when I ask why they tell me it's for a 2.7 gig data that was used, yet the Sim card has been in my purse, so question is who is using the Sim card, and who authorized the subscription they are talking about?