I only know German ones: senfcall.de or bbb.digitalcourage.de

the big ones out there are BigBlueButton and Jitsi. they have a lot of public instances, so you can easily check if they fit your needs.

Has a mobile app!

Stating foss and not even linking the source is … hmm

Nothing realy exept licencing

Maybe consider using forgejo (gitea fork used by codeberg)

And i do think, some changes would be needed but nothing big, also it wouldnt ne activated by default.

Cellphone network calls / texts aint secure at all. If you want to communicate in a secure way you need to use another seevice/app.

Many stated matriy as selfhostable service and i totally agree. Signal/Threema are also good options If you dont want to selhost.