SIP is more or less the signaling protocol for telephone service as VoIP. SIP doesn't include the actual audio, which is usually RTP on a different socket, but this client seems to do audio too.
If you've got VoIP service for a landline, it's probably SIP. I think VoLTE might be SIP too? There's extensions for text messaging over SIP as well.
If you've got VoIP service for a landline, it's probably SIP. I think VoLTE might be SIP too? There's extensions for text messaging over SIP as well.