VoIP can be used in the smallest, right up to the largest of offices.  Adding a phone is as simple as plugging into the network and changing a few settings.

Operational cost of a VoIP service is much cheaper than a traditional phone line, especially for calls over long distances..

With a VoIP system you have the flexibility of having the system configured to operate differently depending on the time.  After hours calls can be diverted to a mobile phone, or diverted to an after-hours message with ease.

Host phone conferences on your system.

Make calls to colleagues within the office.

Have the caller route their call to the right person, with speech recognition or keypad based menus.