(Im sorry gang the PBX section is still a little weak as Foundstone is keeping me busy, you can always check Hacking Exposed)
PBX is short for Private Branch Exchange, a private telephone network used within
an enterprise. Users of the PBX share a certain number of outside lines for
making telephone calls external to the PBX. Most medium-sized and larger
companies use a PBX because it's much less expensive than connecting an external
telephone line to every telephone in the organization. In addition, it's easier
to call someone within a PBX because the number you need to dial is typically
just 3 or 4 digits.