InternetSpySoftware.com

OpenH323 Gatekeeper
Free H.323 gatekeeper (GPL) based on OpenH323.
www.gnugk.org/

Asterisk
Open Source telephony switching and private branch exchange (PBX) daemon. Supported signalling protocols: H.323, SIP, MGCP, SCCP (Cisco Skinny).
www.asterisk.org/

SIPSet
Sip based phone. You can use it as a soft phone, to make and receive calls from your Linux PC.
www.vovida.org/

Siproxd Project
Proxy/masquerading daemon for the SIP protocol. It handles registrations of SIP clients on a private IP network and performs rewriting of the SIP message bodies to make SIP connections work via an masquerading firewall (NAT).
siproxd.sourceforge.net/

isdn2h323
Project homepage for isdn2h323, a Linux-based ISDN to H.323 gateway.
www.telos.de/

Partysip
SIP proxy server. It can operate as registrar server, redirect server and stateful proxy server. SIP is an open standard (IETF) replacement for H323.
www.nongnu.org/

VoIP Billing Software
Offering software that handles billing for VoIP, retail, agent and OSS systems.
www.webvoip.com/

OpenH323 Project
The OpenH323 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.
www.openh323.org/

GNU oSIP
Implementation of Session Initiation Protocol (SIP). This library provides an interface to initiate and control SIP based sessions.
www.gnu.org/

GnomeMeeting - A window to the world.
Telephony over internet meeting software.
www.gnomemeeting.org/