If your ISP is providing you with an Arris router these routers have been known to be problematic with VOIP. Even in True Bridge Mode, the router has still been known to cause issues.
On the DG860A, the SIP ALG can be disabled by the provider. When speaking to the provider you need to ask them to disable SIP ALG & H.225 on any Arris router.
For other models, where the above settings can not be changed.
Option #1: Ask the cable provider for a new router that is compatiable with VOIP or one you can disable SIP ALG on.
Option #2: Put the router in True Bridge Mode (Pass-through Mode) and disable the Firewall. Leaving any service on the router will cause the router to inspect SIP traffic. Ensure everything is disabled. Save and restart the router and then restart the phone.
On the DG1670 router, SIP ALG is applied by default. Any older models that don't give you access to disable SIP ALG have the ISP update modem firmware and disable the SIP and Firewall or fallback to option 1.
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.
See our other article on disabling SIP ALG on other popular routers.