I have run into that, too. It may be transitory.

Sometimes it takes a system reboot to get it (and other functions) to work on the Thales systems. Haven't seen if a reboot helps on a Viasat airplane as we so rarely have to reboot them. Can't really research this on flights I'm flying as we can't use our phones while in the control seat, only our iPads, and the iPads don't ever show the T-Mobile prompt because they are tablets. That leaves me with just my commutes and the occasional deadhead to test it with my phone.