MTN *****s up debit order

We have a MTN contract which started with nashua mobile who closed down so MTN took over. Ever since MTn took over our debit order was a mess, they will take it for 3 to 4 months on the wrong date then They will take all the funds at once. Then all of a sudden they cancel our debit order, now we got handed over for MTN's incompetance. We phoned MTN to find out what is the outstanding amount they say it is R1600 so we paid R500. Then all of sudden we get handed over for R5960.00 on this MTN account. my question to MTN was why do we need to pay this much if the account is not even that much with MTN. We havent been able to use the sim card for a year even thou you pay your account.But dont worry if MTN switches you off incorrectly they will still charge you a reconnection fee. This is absolute bad service. I have never had this problem with Vodacom. After numerous of calls to MTN they now just say sorry phone our legal department.