More and more people are using Voice Over IP (VoIP) - it seems to work well and removes the hassle involved in leasing a PABX or having contractors come in and make updates to the phone system.

From an admin standpoint it means almost anyone can manage basic PABX admin (eg directory management). You also get the joys of being able to plug 'n play your phone (no more repatching every time someone moves desk) and reset voicemail pins along with a wealth of other functionality previously hidden away in the heads of PABX admins. Most VoIP vendors also have cool stuff in terms of messaging integration (eg with Outlook or Notes) and software phones.

We use a MiTel 3300 which is been ticking along quite happily for a few years now with about 340 users (it'll easily handle twice that) - admin is primarily web based and pretty straightforward although it helps to have some basic PABX familiarity before you go poking around with some of the more esoteric options. As its all software based you can just upgrade your PABX to support new generation phones - stick in the MAC address of the phone and it'll boot up with the appropriate software image.

What you need to utilise all of this VoIP goodness is a solid LAN with Quality of Service (QoS) capable switches and VLAN's to isolate your VoIP traffic. We use Cisco kit and its interesting to note that not all of their gear is created equal - implementing QoS on a Catalyst 3550 is much simpler than the 3500XL (slightly older model).

Another essential to save on cabling is Power over Ethernet (PoE) capability in your switches. Some will transparently power your phones and other devices (the newer 3550's and 3560's) while others will require dongles on your phones (the 3500XL's). For older switches (like the 3500XL's) you can also use a PoE 'booster' like a PowerDSine unit and not have to use a dongle.

In terms of basic troubleshooting be very wary of putting your phones through cheap switches and watch the quality of your patch leads. Most VoIP phones share your PC LAN connection - your patch lead goes from the wall into the phone and then from the phone into your PC. The phone itself acts as a QoS switch controlling what your PC does such that it doesn't adversely affect your voice communications - probably not something you'd notice unless you have some other real-time type apps running on your PC (eg intensive Citrix sessions).

Be careful implementing VoIP across a WAN - it can be done but if you don't have the expertise in-house you *really* need to be able to trust that your comms provider will allocate appropriate bandwidth (we allow about 80kb per call so if you have 10 people in a remote office set aside about 1Mb) for a real-time queue and properly honour the QoS DSCP tagging (44 - 46 seems common in NZ).

If you run into choppy voice calls while copying data across the WAN or when more than one or two people use their phone simultaneously then the circuit/routers haven't been properly provisioned and/or your switches aren't properly handling the QoS tagging (your comms provider will point the finger back at your LAN config so you need to be able to show end to end QoS so you can point the finger right back at them :-)

Still - its all worth the pain - being able to plonk a phone down anywhere and use it as an extension of your primary office is a truly great thing.