Disgusting service at Cell C

I had 2 contracts with Cell C and paid them off and closed my accounts. They have not updated on the credit bureau and now it reflects as if I'm still owing them for months and has reflected badly on my credit. Now I cannot get a full mortgage bond because of their incompetence. I've tried calling them a number of times but I hold until I either run out of airtime or until I have to hang up because it's been longer than 40mins. I went to their branch and Campus Square for assistance and they could not assist me because my Sim is now prepaid. I need a paid up letter for both accounts and rectify the ******* on the credit bureau and my name.