They don’t have to! Flat pack doesn’t remove all other ways to install software. But for 95% of use cases, it will do just fine.
Tell this to canonical, they even firefox put in the snap. You know that when choosing “quickly compile something for a flatpack” and “support 10+ distributions”, the developers will choose a flatpack. Which in general looks fine, until you realize that everything is just scored on the mainline of libraries and molded on anything. The most striking example of this is Linphone. just try to compile it…
The only way to connect the SIM number directly is to hack the VoWiFi protocol, but this is not trivial and you still need to install the SIM in the server.
Option 2 - Buy a home SIP2GSM gateway. But it’s quite expensive (by the standards of my region anyway). SMS work with SMPP, calls work too. For goIP I wrote telegram SMS gateway if you interesting: https://github.com/lifespirit/telegram-smpp-bot
Or use SIP providers from your region/operators that support SIP connectivity and then enable full calls redirection. For calls ok.
UPD: or just use VoWiFi from mobile phone. But you need sim slot in phone.
Anyway in all another way you need install asterisk/freeswitch and write config fot it. And linphone client.