Actually, I figured it out, while typing this entry. Apparently, if I enter the number in the form of "xxx-xxx-xxxx" it fails; Omitting the dashes, however, allows it to work. This is, probably, due to code just converting $myphonenum -> $myphonenum@myprovider.
So, this post is not all for naught...
1. Any chance that we could add a line or two to the Nextel code -- either in editinfo_do.bml, or at runtime -- to chomp the dashes, if they exist? It could probably save on tech support issues, down the road.
2. I just realised that messages are not arriving from $user on Nextel, as well -- they are arriving from 'webmaster'