Well, to shorten a long story, I had a debacle of an experience last year, trying to use a little flip phone as a mobile device. It's cancelled now, but I did end up with a mobile phone number.
So, out of curiosity, I pulled up the US Mail online form for a forwarding address, and submitted the cancelled mobile phone number. And guess what -- it went through. So now my mail will be forwarded as before, for another six months.