Unresolved problem for more than a year!!!!!!

I have an unresolved issue with CellC where I was incorrectly billed last year, after a whole long fight to get it resolved, I was assured all was sorted. Now I discover that it is NOT resolved and I have been given a bad credit score because of this one issue that CELLC has caused and NOT RESOLVED!! I am livid!! Trying to take out a new phone contract when this came up - the fact that I cannot take out a new phone anywhere is affecting my business and my work!!! This is ridiculous! I have been trying to get any form of help for MORE THAN A MONTH!!! There has been no feedback and my calls just get transferred from one person to the next. This is such poor service!!!