Billed Incorrectly and refusing to rectify!

MTN have billed me incorrectly in 2014 and after raising the issue, they promised to rectify the matter. Its Mid 2015 and this has still not been rectified, which reflects bad on my credit report. I've also attempted to resolve the matter via e-mail and via the call center to no avail. I ran out of virtual options instead of going to a branch where im reluctant to go as most staff members are *********** that I've came across.