If you've ever been involved with deploying a telephone system, you know it's a big job. Deploying an IP PBX is a big job, too, and it shares many issues with old-fashioned PBX deployments. Those issues include planning, training, and orchestrating the change-over, as well as testing and debugging. But not all of the challenges are the same. Some parts of the process are more complex than installing an equivalent legacy PBX and some are easier.
The first consideration is whether you're planning on a totally new installation (let's say you're moving into a new building) or upgrading the phone system you already have. If you're starting from scratch, then interoperability with a legacy system is not an issue, which reduces the complexity significantly. But you'll still need to make sure your new network infrastructure is VoIP-aware.
You should plan on using a dedicated network for the phone system. Yes, your voice and data traffic could share the same network infrastructure, as long as your network isn't heavily loaded. But at some point, as you begin to require greater network bandwidth, voice quality will suffer and you'll have to split out the phone traffic. You might as well put a separate cabling plant in place at the start, because it costs a fortune to add later. If you already have a PBX, the first thing you have to learn is how you can use it in conjunction with an IP PBX. IP PBX vendors - including 3Com, Avaya, and Siemens - often put proprietary spins on the H.323 and even SIP protocols they use to transmit calls. As a result, out of the box interoperability among equipment from different vendors is not guaranteed.
Most PBX vendors already have a VoIP solution, so your best bet may be to call whoever provided the telephone system you're already using. If you stick with the same vendor, the old and new systems will most likely work as an integrated whole. The other option is to choose a PBX that boasts a broad set of interfaces to third-party equipment. Avaya, for example, says that its gear can work with nearly any legacy PBX.
Those decisions are just the beginning. Ultimately, you'll have to decide who's going to manage the new box. Will it be IT staff or the facilities group already running the legacy PBX? Because VoIP uses standard Ethernet networks, your IT staff will have experience with the network hardware. And because the design of today's IP PBXes is typically more like your existing IT equipment than a legacy phone system, IT staff may also be more comfortable than your current PBX administrator with the new management interface.
Whoever manages the new phone system will confront key issues, such as managing QoS, bandwidth, and the security of VoIP products. Your networking staff will face significant differences between how VoIP traffic and standard data traffic are handled, the biggest one being the sensitivity of voice traffic to bandwidth issues. While QoS should prevent most problems, when the network gets busy, packets can get dropped or delivered out of order. Voice quality suffers greatly when that happens.
The bottom line is, once the basic physical and interface issues are solved, the real challenge you'll face is training and learning how to effectively manage the phone system and VoIP traffic from day to day. You can't skimp on that if you expect your telephone calls to work right. And you surely know what a high priority that is.