I based my test on https://github.com/havfo/WEBRTC-to- SIP/blob/master/etc/kamailio/kamailio.cfg but I blamed my failure on an old rtpengine :)
You need to add 'SDES-off' to the rtpengine_manage strings for calls going to WebRTC. Most browsers don't support fallback to SDES (anymore) and will reject the call if both DTLS and SDES are offered.
With kind regards Pan B. Christensen Developer Phonect AS