Known Issues:

1. SIP ALG is enabled by default, causing intermittent call and phone feature failures.
2. This router is capable of DNS proxying, when turned on it causes intermittent registration failures on Polycom phones.
3. While this device does have the ability to turn off SIP/ALG and DNS Proxy, recent testing has shown that this does not resolve the overall issue.
4. This device lacks the ability to maintain UDP session integrity and therefore cause general call quality issues.
5. Since this device is also managed by the Internet Service Provider, neither the customer nor VoIP provider are made aware of ISP changes.
6. If this device is reset by the Internet Service Provider, original issues will return.
7. In light of the above issues, it is therefore recommended that these devices be bridged to a recommended router.
8. Note that this is the stock firmware for this router. Do not confuse this with the incompatible Arris Comcast Xfinity TG852G or Arris Comcast Xfinity TG862G & TG862G-CT Gateway YN3720.

Resolution:

1. Click on LAN setup
2. Check (enable) DNS override
• Set the Primary DNS Server IP to 8.8.8.8
• Set the Secondary DNS Server IP to 8.8.4.4
• Uncheck (disable) DNS Relay
• If this device is your only router, NAT mode should be set to "RoutedWithNAT". If this router needs to be bridged then NAT mode should be set to "Bridged".
• Click Apply
3. Click on Firewall
4. Disable (uncheck) Ping blocking. 
• This step is needed for to allow the router to respond to our Call Quality Monitoring and Troubleshooting Servers, 64.28.122.110 and 64.28.121.110.
• Click Apply
5. Go back to Firewall. Click on ALG.
• Uncheck SIP
• Click Apply
6. Reboot router and phones.

Additional Resources:

1. Recommended Routers.
2. Recommended Switches.
3. Recommended LAN Configurations.
4. Network Ports and Protocols for HPBX phones.