MTN SA wifi is so bad

Ive received the worst experience with MTN,my biggest mistake was taking the MTN coverage which was a nightmare to keep up with,it worked for only seven days after being paid for,after that i coudnt do anything with it,i cancelled it at MTN a 7 days before the pay date now their call center agents are cutting my calls after i have received an sms about outstanding payment,how do i pay for something that i cancelled in store at Dobsinville mall and took everything back,and the agent confirmed cancellation,i even took it back to them,it wasnt with me,MTN is a scarm i regret ever signing up they messing with my credit score.