MTN billing me for calls i did not make.

I have received large bills over last 2 months when i reviewed itemised billing there were numbers that i didnt call. the call amounts to over R500. MTN claims that since i took out the contract and it is registered in my name i made these calls. when i called to check who owns the phone of the number on my bill, it belongs to a lady i do not know. all these calls were made during working hours when i do not use my phone. I am very disappointed with the service from MTN.