Session Initiation Protocol (SIP) FAQs, background materials and lists of current work in this field. http://www.cs.columbia.edu/sip/

The SIP Center Portal with technical and market resources aimed at commercial developers. Also provides a testbed area for software and proxy development. http://www.sipcenter.com/

pulver.com A collection of information on the protocol. Contains news, reports and an IP telephony job noticeboard. http://www.pulver.com/

Sipcall A UK based service provider. Also contains hardware related links and protocol white-papers. http://www.sipcall.co.uk/

Soft Front Provides a development environment for software centered around SIP/VoIP technology. http://www.softfront.co.jp/en/

SIP Forum A membership based forum. Also contains a news board. http://www.sipforum.org/

PROTOS Test Suite A test-suite to find vulnerabilities in protocol software. http://www.ee.oulu.fi/research/ouspg/protos/testing/c07/sip/

SIPquest Inc. Software component developer for service providers and telecom equipment vendors. http://www.sipquest.com/

Brekeke Software Products for small to medium sized offices and a server database. http://www.brekeke.com/

Parrish Systems Sells off the shelf code for SIP, SDP and RTP protocols and provides a software consultancy service http://www.parrishsys.com

Interactive Intelligence, Inc. Developers of a proxy server. Site contains downloads, documentation and a forum. http://www.inin.com/sipproxy

Curibo German vendor offering hardware and software products. http://www.curibo.de/

Ninetel USA A USA based proxy service provider and supplier of hardware phones and routers. Site contains call rates and service details. http://www.ninetelusa.com

Free IP Call A proxy service provider with information in English and French. http://www.freeipcall.com

SIP411 Directory Services A Yellow Pages style directory of SIP addresses. http://www.sip411.com

VoIP News Information on available technologies and software/hardware product reviews. http://www.voip-news.com

PC Phone Review News and reviews of hardware, software and service providers in the United States. http://www.pcphonereview.com

BaseVoice Commercial software development company that produces PBX, router, proxy and application server software. http://www.basevoice.com

JustIT PBX A solid state hybrid PBX product combining PSTN telephony with Voice over IP. http://www.justit.ws/products/jpbx/index.html

VoIP User Consumer guide and forum. Also has software phone downloads and links to service providers. http://www.voipuser.org