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

Verify there is an internet connection to the Zyxel gateway. If you can access the web (internet), internet connection is active.
Nextiva uses port 5062 instead of the traditional 5060. Enable SIP ALG on the Zyxel gateway and add port 5062 to the "SIP Signaling Port" list.
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 PBX may be timing out causing the phones to lose connection. Increase UDP session timeout to fix this issue. 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. Nextiva provides a list of their VoIP servers here.
Create an address object for each subnet described in Nextiva site.
Create an address group for Nextiva subnet address objects.
Add a policy control (firewall) rule to allow traffic from these Nextiva 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. Nextiva provides a list of their VoIP servers here.
Create an address object for each subnet described in Nextiva site.
Create an address group for Nextiva subnet address objects.
Add a policy control (firewall) rule to allow traffic from these Nextiva addresses to the LAN network where the VoIP phones are located.