So, there are no static IPs. As the UCM sits behind the firewall it has no idea of what the public IP is…unless something, somewhere tells it. As far as a keep-alive, perhaps, again a setting and it may depend on what kind of keep-alive.
The UCM has no idea of what its environment is unless someone programs it so that it does know and subsequently knows how to communicate to the various devices - both internal and external.
Despite the cost, a static is always better as once an IP changes, there is a period that ensues where something must inform the other devices of the change and that change is then communicated to the DNS servers around the world so that providers and remote devices know how to find it.
So, we work with what we have:
- Get a FQDN. Look in your router and see if it supports DDNS and if so, if there are any specific DDNS providers it wants to use. If so, go to that provider and get a subscription for a DDNS account and select your FQDN, again being careful to select a domain the router supports from the provider.
- Once obtained. setup the router with the provider and the purchased FQDN. I use the router for this purpose as if it is the one handling the failover, what better, faster device to let the DDNS provider know of the change so it can then relay the new IP associated to the FQDN to the DNS servers.
- Input the FQDN into the external host field in PBX settings, SIP Settings NAT.
- Check the box for the SDP just undernearh
- Lower down on the page, input the local LAN subnet and any remote LAN subnets that may be connected via VPN.
- I cannot speak directly to the USG and the SIP issue, I briefly looked at the WIKI and decided that after seeing that many posts with questions, I did not have the time nor a device to test with, I will have to leave that up to you.
- If you are not port forwarding and using the firewall to only allow the provider and other known IPs, then you should do so. There is no need to rely on a keep-alive to maintain a pinhole in the firewall and I do not know what the UDP port close timeout is.
- What kind of SIP trunks are you using?
- As your ATT failover is on the router and you mention the internet is still up, the question then becomes what QoS or other means are you using to insure that the normal data traffic does not trample the voice traffic? I assume you have a minimum of 200Mbs with Spectrum, but far less with ATT.
- Yes, in the UCM, Maintenance, Troubleshooting, network capture. This is a Wireshark capture and allows one to see the messaging.
I have a similar setup but I have static IPs on both Spectrum and ATT wireless, but I still use a DDNS FQDN to advise of the change. While my router is not a USG and I am not using an answering service, I have no issues with the failover.