Arris routers have proven problematic with both SIP and WebRTC. Even in True Bridge Mode, the router is still inspecting and changing the SIP packets and interfering with the ICE negotiation with WebRTC.
On the DG860A, the SIP ALG can be disabled by the provider. In addition, ask to have H.323/.225 disabled as well.
For other models, where the above settings can not be changed.
Solution #1: Ask the cable provider for a new router.
Solution #2: Put the router in True Bridge Mode and, very importantly, disable the Firewall. Specifically, leaving the Firewall on, but making every option as permissive as possible did not fix the problem. In the interface, go to Firewall and Firewall Settings and uncheck the "Enable" box. Save and restart the router and then restart the phone.
On the DG860 modems, Sip Translation is applied by default and older version of the firmware does not give you access to disable this or even show that it is enabled. If this is the case you have to have the ISP update modem firmware and disable the SIP and Firewall.
Note: Arris devices with an "M" in the model typically are a modem-only and not a a router and as such, are not a problem.
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