Knowing some history of telephony and understanding the new technologies should prepare you to face your specific business problems. The intent is to help evaluate, select, and deploy future communications systems for at least the next decade.
The goal of UC is to improve communications, which means the mark of success will be increased usage per person. What new functions will a migration add? What effect will it have on network traffic? The direction is up, but where will you start?
Before that increase hits, you should calculate if the IP network is able to absorb voice and UC in addition to data. A PBX will provide some demand information; phone bills are another source.
When planning basic capacity, compare what you know to the assumptions that vendors make when recommending the size and number of servers. Some vendors openly state their basis for a calculation, for example, in terms of the number of emails, phone calls, and IM messages each person makes in a day or hour. If you don't validate the assumptions against your own data, any prediction will be a wild guess and probably wrong.
The same applies to assumptions about costs when calculating a potential return on investment. More on that below.
RFC 5359, Session Initiation Protocol Service Examples, collects best practices for legacy services using SIP “methods” and protocols. Watch for similar documents to emerge in the future, not ...