Determining the size PBX you require should be based on two factors:

  1. The Number of Lines
  2. Number of Extensions

When you add the number of lines/trunks (outside lines) and extensions (handsets, faxes, credit card machines, modems, etc) together you will have how many ports you need from a PBX. While most small offices, small businesses and remote and branch locations will be served well by a small office PBX, some phone-intensive business like call centers may require a larger system.