Bad service

I took online data contract when it was delivered everything was wrong as it was not what we agreed on the sim was delivered on Friday afternoon then I called the marketing company Mondo on Monday and told them to cancel the sim card as it was not what I ordered, then after two days I app**** for new sim directly in the MTN store at menlyn and that was fine working well. The problem started on the billing when I was billed 866 they queried the amount then I was told I have two lines I have been trying to sort the matter with MTN and they are not willing to assist they tell me about mondo and they cannot check recordings of mondo but they can bill me. Please assist with the matter as is messing my credit. *** is my number