If you just want to talk, SIP via Asterisk (server) is OK. Most phones have built in support and there are apps (Jitsi) that works on most OS's. You can also co

If you just want to talk, SIP via Asterisk (server) is OK. Most phones have built in support and there are apps (Jitsi) that works on most OS's. You can also connect it to land-line for calling those without Internet access.