For those who still have a fuzzy idea how IP telephony can help bring down telephone costs, here's a brief article that explains it all. Written by Cindy Waxer for VoIP-News.com, it dumbs down SIP trunking and lists all the benefits associated with it. It also touches the subject of security and ways to mitigate the risks.

Session Description Protocol or SDP describes multimedia communication sessions for the purposes of session announcement, session invitation, and parameter negotiation.

SIP or Session Initiation Protocol is in an application layer protocol used to initiate, manage, terminate and control multimedia communication sessions such as voice and video calls across a network. It typically uses TCP or UDP port 5060 for non-encrypted sessions and port 5061 for encrypted ones.