Bayonne(http://www.gnu.org/software/bayonne/index.html) is the telephony server of the GNU project. Bayonne offers a script driven threaded multiline state event telephony service for building voice response systems and telephony plugins for runtime driver configuration. Bayonne also features "TGI" for making perl applications "telephony aware". Bayonne may be used to build telephony based system administration, home automation, automated attendent, v-commerce, and voice messaging systems.
VOCP(http://VOCPsystem.com/) is a complete voice messaging solution, featuring voicemail boxes, email pagers and DTMF "command shells". Users can navigate the system using a touch-tone phone, leave and retrieve messages (by phone or through the web interface) and execute programs (optionally feeding the program numeric or even text input) on the host machine using the DTMF command shells.
The OpenH323(http://www.openh323.org) project aims to create a full featured, interoperable, Open Source implementation of the ITU H.323 teleconferencing protocol that can be used by personal developers and commercial users without charge.
GnomeMeeting(http://www.gnomemeeting.org/) is an H.323 Video Conferencing application which uses the H.323 protocol. It can connect to a variety of other H323 applications including Microsoft NetMeeting. It also supports ILS servers. GnomeMeeting can work with or without a webcam, and is able to create pure audio communications or traditionnal audio+video communications.
drawboard(http://sourceforge.net/projects/drawboard/) is a Java applet used to make graphical teleconferences (like the whiteboard in Netmeeting). When you draw something on your table, all remote users can see it. It features a client-server architecture and a high-color palette.
H.323 ISDN Gateway (isdngw)(http://www.gnugk.org/h323-isdn-gw.html) is a Linux-based H.323 (voice over IP)-to-ISDN gateway.
Linphone(http://simon.morlat.free.fr/) is a Web phone with a GNOME interface. It let you make two-party calls over IP networks such as the Internet. It uses the IETF protocols SIP (Session Initiation Protocol) and RTP (Realtime Transport Protocol) to make calls, so it should be able to communicate with other SIP-based Web phones. With several codecs available, it can be used with high speed connections as well as 28k modems.
Asterisk(http://www.asteriskpbx.com/) is a PBX (Private Branch eXchange) and telephony toolkit. It acts as middleware between the Internet and telephony channels (like VoIP, VoFR, ISDN, modems, Internet Phone Jack, etc.) and applications (like VoiceMail, directories, MP3 players, intercom, etc.). It has many advanced features such as a codec translation API. The base distribution includes several channel backends, as well as applications. However, the beauty of Asterisk is its ability to be extended using its APIs and dynamic module loader. End users can even write their own applications that run on the system.
The GNU Gatekeeper (GnuGk)(http://www.gnugk.org/) is a full featured H.323 gatekeeper, available freely under GPL license. It forms the basis for a free IP telephony system (VOIP).
JVOIPLIB(http://lumumba.luc.ac.be/jori/jvoiplib/jvoiplib.html) is an object-oriented Voice-over-IP (VoIP) library, written in C++. Its purpose is to make it easy to set up and control VoIP sessions. The session's characteristics can be changed during a VoIP call.
Kannel(http://www.kannel.org/) is an open source WAP gateway. It attempts to provide this essential part of the WAP infrastructure freely to everyone so that the market potential for WAP services, both from wireless operators and specialized service providers, will be realized as efficiently as possible. Kannel also works as an SMS gateway for GSM networks. Almost all GSM phones can send and receive SMS messages, so this is a way to serve many more clients than just those using a new WAP phone.