I have just had the same problem as many others i have had 2 numbers with VM for 3 years or more. I upgraded one number a couple of months back and it was declined at first then they looked into it and accepted it. I have just rang up to upgrade my other number which is my daughters number and was going to be a Xmas present and was declined for the upgrade. I am now considering leaving as this is just not acceptable.
No i was given no reason at all, I was taken through the whole process even chose phone and tariff and was just told I had been declined. This happened when I tried to upgrade a couple of months back, as I have 2 contracts with them. I called them and was told they would look into it and get back to me, I received no call but when I logged on a couple of weeks later my green upgrade banner was there and it was accepted. This time when I rang up to upgrade my daughters contract I was declined again with no reason.