Server Software

Applied Voice & Speech Technologies, Inc. (AVST)
Applied Voice and Speech Technologies, Inc. (AVST) offers SIP integrations to its flagship CallXpress unified communications platform.
http://www.avst.com

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

Brekeke Software, Inc.
VoIP telephony server software manufacturer. Includes product information, free download trials, and details of the support available.
http://www.brekeke.com/

3CX
Developers of IP PBX software for Windows.
http://www.3cx.com

Interactive Intelligence: Interaction SIP Proxy
Site contains downloads, documentation and a forum.
http://www.inin.com/ProductSolutions/Pages/Interaction-SIP-Proxy.aspx

OpenSER
A branch of the SIP Express Router project, offering an open-source SIP server.
http://openser.org/