Customer service nightmare!

My husband and I both have MTN contracts. I never had a problem with them until last year. My husband took out a two-for-one deal and has had billing issues since the day he upgraded his package. From incorrect banking details to debit orders not going off to wrong debit order dates, everything has gone wrong with this contract. He has called and been in the store multiple times. Every time we walk out of the store and think the issue has been resolved, we find out at the end of the month that the same problem persists. This is getting ridiculous, up to the point that they are billing him 3 times the amount of the initial contract. They have greylisted the number, but when we are in the store, they can't detect any issues. Like, what the hell?