Which SBC is right for me?

[diederik]

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.