Thank you for asking this question in the support community. I'm not sure I follow what is happening during your testing. Is the following correct: You have a single SIP trunk configured to the provider, with a primary and secondary SIP server configured on that SIP trunk. When you reverse the primary and secondary IP address configurations in the ADTRAN, the ADTRAN responds with a "404? I think a debug and configuration would be beneficial, as that sounds more like the IP address or DNIS may be incorrect.
The Configuring SIP Trunk Failover in AOS guide provides good examples of this application and troubleshooting. However, if you have additional questions or information, I will be happy to assist you with troubleshooting.
Thanks for the info Levi. What I actually first attempted was just changing the IP address of the primary sip server to lets say 18.104.22.168. I then placed an inbound call from the telco. The 3430 returned a 404 message to that and the telco did not try to reroute the call. I was just easily trying to replicate telco not being able to reach the 3430 primary server and the second call would be sent from the backup server.
What I did to test this scenario was just apply an acl to block traffic from the primary server and with no response the telco failed over so there was no need to try to change the cause code supplied.