Q:

voip / ip phone recommendations

We have an aging digital phone system at my office.  We would like to replace it with something modern, such as an IP based phone system.  I have read a bit here and there, even browsed through some voip beginners books.  We have a few quotes with local vendors for IP phone systems, but if it can be done in house, I think we can save a lot of money and get many more features.  
I am noticing that all of the features of these systems require add on licences.  I have heard of Asterisk, and see that it's implemented in both Zentyal and Clearos.  I am a capable Linux admin, but I know next to nothing about voip.  
So, I am looking for sound advice from people who know their stuff and/or have been in the same place and found/installed a FOSS solution that worked.  What hardware recommendations are there?  What is the best way to handle QOS?  Do we need seperate network runs to each phone and computer, or can there be one network run to each station with a l2 switch and have QOS handled at the backbone switch? 
My current network: 
-We will be getting a new EOC (ethernet over copper) line installed with 5 mpbs up/down within a month.
-We have all unmanaged netgear switches, and will be separately upgrading to a pfsense based firewall to connect WAN to LAN.  We will be upgrading to whatever managed switch for the backbone is decided to be adequate.
-There are about 30 user computers total, with 5 servers.  Three servers running debian linux and two running Windows server 2003.  

A:

ServerFault doesn't really do shopping advice - you need to consult with vendors and decide what system works best for you.
Some points to consider:

Do you want a fully VOIP system (IP within the office, your PBX connects via VOIP to a provider that connects you to the PSTN), or a hybrid system (IP within the office, and your PBX connects to the PSTN directly)?
Do you want integration with Outlook/Exchange?
Do you want a vendor-supported solution, or are you comfortable with a more DIY approach?
What features do you want on your phones (voice, video, multiple lines, color screens, etc.)
How much are you willing to spend?

Some of the other points you bring up:
QOS
Depending on your network you will need to implement some kind of QoS (internally and/or externally) to ensure that VOIP traffic isn't delayed (latency/jitter is a KILLER).
Infrastructure
Many (if not most) VOIP phones have a built-in pass-through switch and support VLAN tagging, so you can run a single piece of copper to the desktop to service both the phone and the workstation, however note that the bandwidth consumed on that line counts against the port's maximum signaling rate regardless of whether it's voice or data traffic.  Users who move large amounts of data are candidates for a dedicated network drop.
Experience
Telephony (even VOIP) is a specialist niche: Your company probably considers telephones a business-critical must-work-110%-of-the-time system.
You say you're a capable Linux admin but know next to nothing about VOIP -- This tells me you should work with an expert to design and implement a solution that meets your company's needs in a reliable way, then ensure that you are proficient in its maintenance.  Taking that approach will let you learn about VOIP technology without making potentially costly mistakes that can create havoc for you and your company.

