MTN have bad service all the time

Good day. I had a bad service at MTN store for 3 months now. First time I was at the store on the 2nd November I went to took 2 cell phones contract I was assisted by Ayanda Danisa. I supposed to pay R299 and R349 for 2 cellphone. Every month they don't debit, when I ask why they not debiting they telling that I need to pay R2400 every month for 24 months which I didn't sign up for and. I took a phone that cost R6749. I need help because I don't even know where is the rest of the money coming from because the contract said I will pay R648 per month. I always call MTN and go to the store all the time but no one assist me. This is already affecting my credit profile as if I am a bad payer. I call MTN with due respect but they end up hanging the call on me while I need help. Please assist me as soon as possible. For November installment I had to pay that R2400 but for December I don't see why is there a need to pay such amount. They I used spent limits which I didn't even receive in December because they didn't put them and I didn't use the other Sim card but they still saying I need to pay R2400