[FAQ] Having issues with VoIP/SIP traffic with 8x8. How to troubleshoot?

Verify there is an internet connection to the Zyxel gateway. If you can access the web (internet), internet connection is active.
8x8 uses port 5060 & 5061 (5060 is primary) for phone registration
Verify phone IP settings to make sure it has an IP within the Zyxel gateways LAN subnet. Phone should have a valid LAN IP address, subnet mask, gateway address and DNS server(s).

UDP session with VoIP service may be timing out causing the phones to lose connection. Increase UDP session timeout to fix this issue (e.g. 3 minutes or 180 seconds). Additionally, you may want to verify with phone manufacturer/provider to see how often the phone checks registration and increase UDP session timeout on Zyxel gateway based on this info.
Create a policy control (firewall) rule to allow communications with VoIP provider servers. Depending on geographical zone, 8x8 provides a list of servers on their "Virtual Office Technical Requirements" pdf guide found here. We will use US West Coast servers for this example.
Create an address object for each subnet described in 8x8s site.
Create an address group for 8x8 subnet(s) address objects.
Add a policy control (firewall) rule to allow traffic from these 8x8 addresses to the LAN network where the VoIP phones are located.

Confirm your network is not in a Double NAT. A Double NAT means the device in front of the Zyxel gateway is also a router. Many service providers assign a modem/router combo appliance. To verify if this is the case, do the following:

Open a web browser and go to https://myip.dnsomatic.com/ or https://domains.google.com/checkip.
Verify the address with that shown on the Zyxel interface status option in the Monitor menu.
If addresses do not match, then the device in front of Zyxel gateway is a router. In this case you will need to contact the service provider who assigned the modem/router to you and ask them to bridge the device, so it functions as a modem only.
Repeat steps a & b after the modem has been set to a bridge state to verify you are no longer in a Double NAT.

Enable SIP ALG if option is currently disabled.
Create a policy control (firewall) rule to allow communications with VoIP provider servers. Depending on geographical zone, 8x8 provides a list of servers on their "Virtual Office Technical Requirements" pdf guide found here. We will use US West Coast servers for this example.
Create an address object for each subnet described in 8x8s site.
Create an address group for 8x8 subnet(s) address objects.
Add a policy control (firewall) rule to allow traffic from these 8x8 addresses to the LAN network where the VoIP phones are located.
Enable bandwidth management on the Zyxel gateway and provide SIP traffic priority on the network.