MTN Accounts

I have been loyal customer of MTN for 21 years. I have tried to do my Upgrade with the iStore for 4 days. Every time they try and do the credit vetting it fails due to an error on MTN side. I have phoned numerous times and every time I get a different reason why. Then I get told that they problem is fixed only to find out it hasn't. I keep on phoning the accounts dept number and I wait 10 mins and then get cut off!