Hi ghost,
nethserver-freepbx and FreePBX packages has been under active development and there are a few changes already released on last packages you can find on nethserver-testing repository:
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Now, under the tab PBX Access on NethServer Web UI you are able to easily open firewall ports for WebRTC (TCP 8089 and RTP ports UDP10000-20000)
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FreePBX web interface can be accessd only from green interfaces by default, you can change this behaviour adding networks from same UI page
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About WebRTC, you could also try Janus, a gateway that uses normal sip/pjsip extensions on FreePBX side and WebRTC between browser and Janus https://github.com/meetecho/janus-gateway . There is a nethserver-janus package on testing repository.