SIP is the right protocol. I'm just saying you don't need a SIP library, but a SIP proxy/server. In a similar way that when you want to handle some web requests

SIP is the right protocol. I'm just saying you don't need a SIP library, but a SIP proxy/server. In a similar way that when you want to handle some web requests you want for example Apache and not LibHTTP. OpenSIPS will do what you need. But configuring it requires a bit of reading. There's no easy way in the VoIP world unfortunately.