Hi,
In theory it will work, but it might not be a very good experience because as you are probably aware Atom cores are quite weak. They will be responsible for running the NAS operating system and services, as well as running a VM on top and that might be a bit too much for it.
A dedicated device (even if a Pi 3B+) should be a better solution in this case since it won't have to do anything else other than run the PBX and has already been tested by us for upto 16EXT and 8SC.