commercial system (Score 1)

It sounds like you will want to use some kind of commerical system I have used asterisk and its a good product, but for a fair sized phone system you will want to go with either a Cisco or 3com solution. The company I work at sells and installs 3com phone systems. The 3com systems are relitivly simple to install and can be easily expanded to handle as many users as you need, you can choose to use regular analog lines or a T1 trunk for you incoming phone lines. Although I work with 3com systems all the time and prefer them I would encourage you to look at both Cisco and 3com and choose a system that suites your needs.