Hey, that looks really cool. I've been wanting to mess around with this stuff on PinePhone, which is in a unique position here since there are third-party images for the baseband which are mostly open source[1].

I've been especially interested in trying to reverse engineer what's going on with Google Fi on Android, but it is definitely a bit over my head, given that until recently I didn't even really know what an AT command was :) I'm guessing since it's Google the carrier stuff is mostly for fallback and all of the actually interesting stuff is done using protobufs over a data connection. (Fi is also interesting because you can make phone calls on the web over WebRTC. I wonder if that's some kind of gateway to SIP, or what.)

[1]: https://github.com/the-modem-distro/pinephone_modem_sdk

(Note: I didn't dig deep enough, so it wouldn't surprise me if the blobs in this are actually where the interesting stuff happens, but it's still probably a great base for experimentation nonetheless.)