Over charged debits

I took a contract with MTN for the amount of R299 and after three months this service provider decided to debit me R1600. Without a warning or any reason why as to they would charge me this amount ! To get a hold of the fall centre is a mission because the robots that respond to you can't give you any kind of solution to the problem. Is ashame that you out your trust in a service provider like this and they don't deliver , really poor customer care and I am sit waiting for a solution to this problem.