Open Source PBX (Asterisk), or other options for worker coops

A worker coop in Tokyo is planning to move to a new multi-floor office building and we are discussing what kind of alternatives are possible, instead of copying the current system of office phones (20 telephones, I heard) attached to the PBX.

What kind of office phones do you have? Have you gone completely mobiles phone only? Do you just attach a USB “phone” to your computers?