Alkahest has an interesting bit on the ‘pros and cons’ (but mainly cons) of Voice over IP in what sounds like a large oganisation when you have to switch over to using old kit that doesn’t support QoS in quite the way you need.

QoS or Quality of Service is a prioritisation concept for network traffic which is especially critical for Voice over IP. If your network packets can’t get through then neither can your calls.

The point is also made that your VoIP suppliers, if the must also be your support, must be good at actually helping you rather than just selling you products.