We use the Cisco IP Phones & Service.. (Score 5, Interesting)

I work for a company that has about 12 people and we use the Cisco Systems IP Phones. They work pretty well, have all the features of a normal PBX including intercom, call transferring, etc and they're relatively cheap.

The cool thing about these phones is each phone gets its own real phone number as well as internal extension. We are located in California and when we have trade shows in Florida we take one of these phones and plug it into any ethernet jack. The phone auto-configures itself and you get the same phone number and extension and you can call other people in the office on speaker as if you were in the next cubicle. Pretty rad. Hope this helps.