They still make dumb phones. And you can get a voip client on Linux.
Many so called dumb phones are just Android phones with buttons, real dumb low powered devices are being phased out because you don't get support for VoLTE Emergency
For voip, I recommend jmp.chat and cheogram. I've been using it for years now.
Many so called dumb phones are just Android phones with buttons, real dumb low powered devices are being phased out because you don't get support for VoLTE Emergency