Mtn making contacts more after a few months

Very bad services. I am paying the same contract for about 1 and a half years, now when i got an extra contract with them, now my first contract is suddenly more than i used to pay plus the new contract. I went to a branch, they told me that mtn is for some reason over charging me, they said they emailed them, now they suspended my number because i didn't pay the extra amount. When i call them they only say they can't assist me with my problem