Unsatisfactory service

I’ve been receiving calls and emails from debt collectors for an account that I’m not responsible for, I upgraded at month end, and was double debited albeit having paid up my account in full before upgrading last year February, however even after several calls to mtn help desk and numerous visits to mtn pavilion I’ve still had no joy. Staff at mtn namely Ramona gave me incomplete advice while I was upgrading, she didn’t explain the split in mtn to mtn calls. And the information she gave me was untrue. I was promised a call back by the manger of the store, however even that hasn’t panned out. Service at the Pavilion store left and continues to leave a bitter taste in my mouth