External IP's and network failover to Cradlepoint

I have to set the ExternIP address in FreePBX to our normal Static IP given by our Internet service provider. Also, I have passed to my sip trunk provider, ZTELCO, two external IP’s, one for my normal provider, and one for the cradlepoint backup, so they can allow access to their trunks. Both the local provider and the cradlepoint are connected to a Cisco 5512, which also connects to a local switch. I do not know if there is a way to set up the PIAF Pbx to work on the fail-over to the cradlepoint without going in and changing the externIP setting. I was trying to explain to my IT guy, but it was confusing him. I assume the IP in the externIP setting is used in the SIP header to tell the remote end the SIP reply-to address. The remote doesn’t automatically respond to the real TCP/IP it received the invite from and on failover is still trying to send it to our normal providers Static IP and not the cradlepoints. Correct me if I’m wrong please.

Does anyone have a way to keep the PBX running with an external SIP provider in a network fail-over scenerio without having to change the externIP in FreePBX?

I see several other posts on same subject. Will rifle thru to see if I can find acceptable answer. Local DNS/APP that updates FQDN to resolve to current externIP sounds plausible.