I happened to miss my call a couple weeks ago when I was in a show at Busch Gardens. When I saw the number I reverse-searched on Google and saw a few results showing Royal Caribbean. It was with that I determined the number was them, called back, said I was hoping it was an upgrade call I missed and am returning. They had to do some research since they are so big but ultimately was transferred and assisted with my upgrade!
With that being said, I can't guarantee you'll have the same results, but if you do miss the call/ignore unknown numbers and get back pretty quickly, it is not impossible to get the upgrade if you were indeed called. Good luck!!