Free VOIP / SIP phone. Use your PC as a phone. Works with any SIP/ VOIP provider/ phone service. Easy to use interface to transfer, forward calls or set-up conference calls. Free download from http://www.3cx.com
Free VOIP / SIP phone (softphone) for Windows. Use your PC as a phone. Works with popular VOIP providers and SIP PBX's (including 3CX Phone System). The interface is easy to use and similar to that of popular Smartphones.
Make FREE pc-to-pc, cheap pc-to-phone, pc-to-fax, phone-to-pc, fax-to-pc, SIP-phone calls over Internet or ISDN/PSTN telephone networks. Use your computer as internet telephone, USB phone, SIP telephone, ISDN phone, FAX, Voicemail.
Ozeki VoIP SIP .Net SDK allows to develop C# VoIP softphone to make/receive voice and video calls. It supports basic telephone functions like make/receive, reject, hang up, hold calls, call transfer, DTMF handling, HD video, extended codec support
SIP tester is a free VoIP load testing tool which enables you to test VoIP network, SIP software or hardware. It is able to simulate thousands of simultaneous incoming and outgoing SIP calls with RTP media, analyze call quality in real time.
Collanos Phone is a software-only Voice over Internet Protocol (VoIP) phone and a multi-protocol instant messenger supporting Jabber, MSN, ICQ, AOL, and Yahoo! protocol combined in one application. It is available on Windows, Mac OSX, and Linux.
Ekiga (formely known as GnomeMeeting) is an open source SoftPhone, Video Conferencing and Instant Messenger application over the Internet. It supports HD sound quality and video up to DVD size and quality.
Billion PBX - new generation phone system working in IP-networks using SIP protocol. Billion PBX provides not only voice calls but also a whole range of additional features like call forwarding, voicemail, callback and many others.
VoiceXML Gateway for SIP based VOIP service, traditional PSTN phone services, and Skype. TAPI development API for interactive telephony application. High level development platform based on W3C VoiceXML.