The OpenPhone Project: http://www.openphone.org/
The RTP Specification (RFC-1889): http://www.ietf.org/rfc/rfc1889.txt
Excellent RTP site by Henning Schulzrinne: http://www.cs.columbia.edu/~hgs/rtp/
Telephony signal encapsulation in RTP data types: http://search.ietf.org/internet-drafts/draft-ietf-avt-tones-00.txt
OpenH323 Project: http://www.openh323.org/
The SIP home page: http://www.cs.columbia.edu/~hgs/sip/
The MGCP Specification: http://search.ietf.org/internet-drafts/draft-huitema-megaco-mgcp-v1-03.txt
Vovida Networks—RTP/MGCP/H.323/SIP Libraries: http://www.vovida.com/content.cfm?pagename=opensource/
General Linux Telephony: http://www.linuxtelephony.org/
Voxilla—the definitive source repository for Linux Telephony: http://www.voxilla.org/
IETF Working Group on IP Telephony: http://www.ietf.org/html.charters/iptel-charter.html