@Tommy You might look at https://jmp.chat/
I got it for pure XMPP access to SMS/MMS, but later got their SIM and a data plan. They're a MVNO--T-Mobile here--and have a fork of Android Conversations to also do telephone calls (based on WebRTC, apparently).
So it might help, though of course I recognize an Android app isn't an immediate solution for UBports! But they do support SIP into telephony for your phone #.