Tried freepbx on debian on orange pi PC, whose CPU same as nanopi Neo. works! but just a toy.
Moreover, do believe “20 simultaneous calls” too much for a weak CPU like H3 or the CPU on raspberry Pi.
If you plan to deploy a production PBX, don’t waste time on ARM structure SBC (some high-end SBC might be good, but expensive). Hence, try to find used x86 PC if the budget is an issue for you.
The advantge of x86 PC obivious:
install freepbx distro directly;
commerical module available;
powerful enough to handle “20 simultaneous calls”;
also process fax well.
In my opinions, a used x86 chromebook much better than ARM SBC. see my post: https://community.freepbx.org/t/most-cost-efficient-pbx-install-freepbx-on-a-chromebook/72363