In case people are interested in existing open-source WebRTC softphones that can connect to Asterisk: I have been working on one the last year(https://github.com/garage11/ca11) that uses SIP.js to call over a PBX like Asterisk. Lately I added video conferencing support that relies on Asterisk 16 SFU. There is some additional gathered documentation how I configured Asterisk (https://docs.ca11.io/developers/asterisk) myself to make it work. There is also a PWA version running live (https://ca11.io/) in case you want to try without building CA11 yourself. Needs an Asterisk installation though.