bad service

I have a contract with mtn ,the amount that was agreed on was R1050 every month on the 20th so yesterday they decided not to deduct money from my account Today I get an email that i am an installment behind on my mtn contract,they saying that my bank rejected the debit order yet they managed to deduct the R87 from my bank account which i do not know what is it for yet they want to deduct 1050 on top of the 87 that they took...mtn is a ****