Cell c is the worst service provider

I opened a fibre account with cell c, the guy came to my house and we filled in the forms and added the bank details and selected debit order. Whoever captured the details at cell c selected the cash option, so is not knowing this, thinking the debit order is going off, because our fibre line wasnt cut.. and 3 months or so passed then cell c contacted us and said we owe them over a 1000, I called in and insisted on speaking to the supervisor because the call centre agent was rude and ot helpful, and he did some research, and said he is gna request our contract, I personally think he was hoping I didnt select debit order on my forms, but to his surprise I did, and he realized it was their fault. Now the biggest problem here is that this mess up affected my wife's credit profile, and it stopped her from being able to apply for credit. I called in and the supervisor made all kinds of promises only to turn around and tell me something different.. when you owe them money they quick to request payment and black list you, but when they f##k up, they cant do anything to help. Please advise me on how I can take this further.. because I cant let this go.