Ongoing problem for a year- upgrade denied

I have been a loyal customer of Cell C for 22 years and have done 10 upgrades over the years. I have 3 contracts.I have never missed a debit order and have never been in arears. I now have an ongoing problem with Cell C for a year. The assistant at the branch has been trying for a week to get this sorted, but nobody else is willing to help. There is an account code against my name since last year August for an arears amount that is not my debt. My number was deactivated last year in August. I went in to a Cell C branch and my number was activated immediately as my account was not in areasrs. A month later I got handed over to a debt collection agency for this outstanding amount. I sent a statememt to the agency indicating that my account was not arears and it took almost 3 months to get them off my back. I am elligible for an upgrade now and my upgrade was denied because of this account code on my name although my account is fully paid up and not in arears. It is more than a week now of emails and trying to sort this out, but to no avail I am very upset and disappointed in Cell C and their service. I am at the point where I want to let my contracts finish and then move to a different service provider, but I am willing to stay if this issue is sorted out timeously.