I have been spending a TON of time in the recent months working with voice over IP of all different types - here is what I learned.  Asterisk is an enterprise ready software package that can do everything you ever imagined a voip phone system could do.  You can have phones in remote offices 4-digit dialing, you get really cool voicemail, call routing, hunt groups, ring multiple places at once, forwarding rules, complex interactive menus, and much more.  We at Dr PC Fix are switching to one, and porting our phone number to a provider that was found online.  So far our experience with them is limited, and actually after-the-fact we found a similar provider with VERY high praise on the net.  You can build yourself a server to run this package for pretty in-expensive if you want (like $150) but for business use you should really buy more business class hardware.  If you buy a phone system ready to run from us you get warranty and some support built in too.

Our setup will look like this:  Incoming phone number, IP to our PBX onsite, then you get our voice menu instantly answering - it will give an option to instantly get transferred to a tech (in our office, or cell phone - automatic) or talk to someone about a non-emergency that is there in the office.  The phone system does track phone calls, and what buttons people pressed, with all callerID information and call times so we can go back to do proper billing for phone calls. 

Pretty cool!