As a business that size, I would not invest in a phone system. I would use something like grasshopper.com.
Asterisk is nice, and i've used it for systems fielding thousands of calls per day, but frankly, who's gonna take care of it in a business that size? What do you do when your internet/phones go down? As a small business you're not gonna have the best internet connection either, and by that I mean service with a 99.99% uptime. You'll have regular business dsl/cable with a shitty 24 or 48hrs policy, which could mean your phones would be down several times per year. Phones are something others expect to just work.
I've use grasshopper before with good results. You will still need some actual phones, but it will provide a menu system, music on hold, and other stuff. You could use it in conjunction with a voip or regular landlines, but it provides the front-end, voicemail, menu system, and is quite inexpensive. If you ever get big enough, you can deploy your own systems in-house and migrate off of it quite painlessly.