There are a number of good sites that publish support information for Voip, particularly related to Asterisk and the variants that we use.
They are listed here:
Info
Basic Asterisk Primer written by Kodak on the Trixbox forum.
Dumb Me guides for Trixbox and Asterisk @Home
Asterisk @ Home Handbook A very comprehensive handbook, this is still popular at the time I am writing this the link at the top of the primer reads This page has been viewed 526483 times since November 1st, 2005
Trixbox Handbook.
Tutorials and Projects
See the tutorials at Nerdvittles here.
How to get the UK weather info on Trixbox. link referenced on this Trixbox forum thread.