FreePBx Dual Home for Extensions

Hello dear Asterisk community,
I have a small challenge here that I have been struggling with for days.
We have a network of several FreePBxs that are connected to each other via Dundi trunks, with an assigned IAX2 trunk and the appropriate outbound route.
We use Yealink T46s as telephones.
Thanks to the Dundi trunks, I don’t need a hidden number plan, but the systems communicate with each other after an invite to find out where the extension is registered. The whole thing works wonderfully, all participants can call each other without any problems.
The Yealinks have the option of being able to access a backup server at the registrar. And that is exactly what I am trying to do right now.
My goal is to store identical credentials for the extensions on FreePBx A and FreePBx B, so that every device can log on to either A or B. Never at the same time.
Unfortunately, I have not been able to do this so far.
If a subscriber from PBx A tries to call a subscriber who is registered in PBx B, PBx A immediately reports that the line is busy because it recognizes that the extension is configured with it but is not online.
A rule in the advanced settings of the extensions (->Advanced ->Optional Destinations → Unreachable) to forward a phone number to a main line if it is unreachable unfortunately does not work either. It does not matter whether I select the IAX2 or Dundi line. No request is received in the neighboring systems.

Does anyone have a good idea how I can configure two systems in live operation as mutual backups?
Thank you very much and best regards!

First and foremost you don’t allow some users to be on A and some users to be on B. This is going to be problematic and cause issues with various items such as backups and even with certain states of calls and/or devices.

This should be setup in a way that everyone uses A until such a time that A is unavailable for whatever reason and then they switch to B during the outage.

(Wow ! somebody still Dundi’s!)

In 2024 You might want to look into moving to SRV records in your ‘Name Service’ for both load balancing and failing over, if they point to raw SIP proxies not FreePBI, it should work even better in failing-over states with fewer dropped calls

Thank you very much for your answers.
Unfortunately, I don’t have the ability to work with DNS entries. Everything runs via IP addresses in a self-contained and protected network. Having all participants only on PBx A and, in the event of a fault, on PBx B…how is this supposed to work fully automatically on the SIP level? There is no way to manipulate DNS entries or IP addresses in the event of a fault.

If indeed an air-gapped network, perhaps this will help

bind is a fully adequate NameService for SRV records.

i would just use two lines on the phone, line one goes to PBX A and line 2 is registered to PBX B, two separate FreePBX servers, one is A and the other is B
both can use the same setup just different IP address.
if you call extension 800 on line one it rings through PBX A and if someone calls on line 2
it will go through PBX B
i have done essentially the same thing by using an existing image of FreePBX and putting it out on a new raspberry pi at a different location on the internet

This breaks so many things as I’ve pointed out. It’s not really a backup or redundancy, it’s just a phone on two different PBXes.