Which SBC is right for me?

[PedroPG]

I don't need everything that Rockpro64 offers and that some models are out of stock, I will study which one may be the most suitable for me.

I would recommend to NOT discard models which are currently out of stock. Most can/should get in stock in some time; it won't be out of stock for as long as you've encountered with RPi's ;-)

If you provide more precise hardware requirements, people can give you more precise recommendations.
You probably know what things are important for PBX functionality while most here probably don't. They may make a somewhat educated guess, but knowing is better then guessing.

What you can and should do in any case: check the wiki pages for the boards you're considering. They are pretty great and also tend to document if something isn't working as well as one would expect.

I don't rule out models currently out of stock... Also, other issues prevent me from devoting my time to this now and in the meantime some models may be back in stock.

For a PBX I only need an SBC that has an ethernet connection, RAM memory (2/4 Gb), an SD/microSD card (32 Gb, no more) for Debian 11 + IncrediblePBX from which I can boot the system and the PBX, a cell/battery that maintains time and date; and that it starts automatically in the event of a power failure.

I don't need a monitor, keyboard, mouse, WiFi, Bluethoot, USB, SSD, GPIO...

My colleague implemented the first PBXs back in 2014, when the usual Raspberry model I think only had 512K of RAM... With the current versions of the OS/PBX you will need at least 2 Gb of RAM, I suppose.
I think it could be done with the current Raspberry PI 3, using older versions of the OS/PBX, but I'd rather be able to use updated versions of everything and so look for something better than the PI 3.

Sorry I can't explain the technical requirements better.

Greetings.