www.sip.us/sip-vs-voip-whats-difference/

SIP vs. VoIP - We explore the definitions of SIP and VoIP and uncover how they work together to deliver low cost telephony for business.

www.voiptoday.org

We are living today the dream of yesterday. VOIP Today magazine is a freely available and independent online publication presenting up-to-date VoIP news and information covering all aspects of the VoIP technology, internet telephony solutions, networks ...

nerdvittles.com

A blog on PBX and VOIP. This also seems to be home of Incredible PBX for Raspbery Pi.