A couple of Observations:
1) POS to me means at least 2 things -- 1 is a "Point of Sale" terminal (think cash register in a store) or 2 -- a derogatory term (I wont go there). I've heard the term "POTS" (Plain ol' Telephone Service) which refers to the ANALOG lines - non trunked, single lines.
2) Going into a VoIP environment, there are a lot of benfits -- but it also may require some changes. If you are expecting to "trunk" several lines into your business (call one number, can be routed to several different lines inside the office, etc) what you may want to look into is a VoIP oriented PBX (something like
http://www.asterisk.org/ ). I would suggest that you get a "professional" who has expereince in VoIP traffic and PBX's and the like to get involved. Maybe Vonage can suggest a local expert / vendor to help out. This also may affect your broadband capability needs. More than likely, you may have to switch out PBX equipment for more VoIP friendly equipment.
3) I would do this in "measured steps" .. especially for service crossover processes. like:
a) Get one or two Vonage lines in, get a PBX running, and get used to the operation of the service
b) schedule a cutover from the BS lines to the VoIP lines.
c) ensure the VoIP lines are working, cut the BS lines.
d) make sure you have an alternate method to get to voicemails just in case things hit the wall (like have a copy of all voicemails go to an email account that you can run out to a WIFI hotspot and retrive -- and then return calls with a cell phone .)
I've recently been invovled in getting a company (50 folks) moved over to VoIP using Cisco equipment. It's very cool, and I wished I paid more attention to what went on. I can tell you that the company in Augusta talked A LOT to the parent company in Sweden. Moving to VoIP saved them a BUTTLOAD in call costs. But there was some significant investment on the front end to get things running.
good luck and keep us posted. VoIP is definitely the future of voice telephony.
Mike
PS -- a Google of "Asterisk Vonage" turned up a BLOOM of articles. Check it out