Poking around the forums, it looks like this was still an unsolved problem, so I sat down with Claude Code (I know, I know, sorry) to see what we could come up with, and long-story-short, we got it working:
Incoming/outgoing calls
Incoming/outgoing SMS
Data
It's simply running on Ubuntu Touch 24.04-1.3/Android 9
I'm on Verizon (US) here, so other carriers may vary.
If someone's already got this working, my apologies for cluttering up the forum!
I'll be honest, Claude's telecom knowledge is way better than mine, which is how we finally got it working, but I can answer any technical details and share what we had to do to get it working. Right now, there are some manual hacks on my phone, but the basic story is it took a few changes in qtibinderpluginext.so, starting up some daemons, and a change to /etc/ofono/binder.conf
I'm happy to coordinate with anyone who knows a bit more than me about how to incorporate this into UT.