Usually, there is an already existing PBX dial plan and IP Telephony is to be integrated into this dial plan. If the existing dial plan has a free numberblock, then the first approach would be to give IP Telephony the whole block.This makes the configuration very simple because it allows prefix-based routing (see Section 4.1.1.1.1).The problem is that either only new telephone users get an IP telephone or that every user who wants to use IP Telephony gets a new phone number. So this approach is not suitable for a seamless migration towards IP Telephony.