In this NetworkWorld Special Report, there are a collection of articles describing the process of planning for and Migrating to IP Telephony. The articles include:
Making the Case for VoIP
Pain Free VoIP
ShoreTel Brings High Fidelity to VoIP
Vanguard Tackles VoIP, App Performance Management
VoIP Requires Wide Range of Planning
Download this document to get a great introduction to the issues you will need to consider in planning and migrating to VoIP.