What authenticates Sangoma phone dpma apps and phone to freepbx server?

I have 2 FreePBX servers. They are configured separately, but identically. I’m using 1 as a cold spare.

The Sangoma phones work with both, but only if I factory reset the phones when I move the phone from 1 to the other. If I don’t factory reset the Sangoma phone when I move it over, it complains at boot time that the server is somehow wrong. They know that the server isn’t the correct one.

Both servers have the same IP addresses. Both servers have the same phone extension, same phone users, same phone secrets. What else is there?

Is there some further authentication that the phones use/receive when they connect to the server the first time that I could copy to make them work with either server without factory rebooting them each time? Only 1 server will ever be connected to the network at the same time, it is a cold spare.

If I don’t reboot the phone and simply unplug 1 server and plug in the other server, so the phone doesn’t reboot, the phone still loses app connection to the server. So, it’s not just during boot where it doesn’t work.