Javascript SIP library sip.js and JsSIP differences?

Question

I have three doubts that require some clear explanation .

1. A clear purpose of those two JavaScript SIP library in relation to Webrtc and sip signalling.

2. Difference between Sip.js and JsSIP JavaScript SIP Library.

3. Does IP PBX means sip server or sip proxy as well

Answer 1

1. Those are WEBRTC SIP libraries, the purpose is to add voice and video communication to a web app.
2. No major difference. both have more less the same features.
3. IP PBX means a business phone system, and a SIP server is the software that send the call to the phone device.