the big advantage of VoIP is that most of its engines is software based.
Besides regular switches, routers and gateways you have to -technically- only look at your firewalls and your T1 connection. A very important point if you want to have remote agents, or if you are planning to offer some carrier services to your customers, is the delay rate.
But in addition to these technical issues, it’s much more important to take a close look at the strategically important parts, like the quality management, the HR-planning and training process and the very important changes in the managament because of a different workflow and a different approach to the business.
Best wishes to your projects.