Cell C not doing their jobs and customers keep on paying for their mistakes

I requested for 3 numbers to migrate to prepaid on the 1st of May. 2 Of these numbers were migrated, but the 3rd one was not done. Cell C conveniently keep on billing me for the number that should have been cancelled. They refuse to do this back dated even if it was their error. This is a joke of a company and I need my account credited for May and June,