My guess would be something from the mobile linux stack, but that might be assuming GSM is used directly, not controlling an external android or something like that which Im guessing youre looking for?
Anyway it seems that for example PostmarketOS uses https://invent.kde.org/plasma-mobile/spacebar Theres prob. more alike. And when everything else fails, its pretty much guaranteed theres some XMPP magic to do that, there always is.