Mtn upgrade declined without any reason or response to the query

I have had two contracts with mtn for between 7 and 5 years, I went under debt review years ago but have upgraded both in that time twice. I am not in any arrears with mtn yes my upgrade was declined with no explanation, I've made 12 calls to customer service and every other dept they have put me through to, I went to different stores and contacted mtn head office but to date no one can tell me why it's being declined. I have been contacted 4 times by the upgrade dept who at the end of the calls say its declined. I have emailed my query twice and still no contact has been made. Surely after 7 years of being a contract customer and never fallen into arrears someone would be willing to assist?