I still don't understand why the phone even needs to be activated on the dumbphone line, anyway. When the phone arrives, can't it just be activated on the line with the UDP? I don't believe the phone is shipped with information already tied to the dumbphone line, only activating it on that line will make it tied.
Bingo!
---------------------------------
The only reason I can think that the sim card can be flashed is because the CDMA/LTE is both contained in the same chip, thus allowing the nano-sim to be flashed, this is all just speculation on my part and in no way the authority on how it is.