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

Graphical VoIP software for the Linux Gnome desktop. Includes RtpTunnel utility for tunneling RTP UDP-based traffic through a firewall.

H.323 compatible videoconferencing and VoIP application to make audio and video calls to remote users with H.323 software (such as Netmeeting).

A free, scalable telecommunications application server by the GNU Project.

Implementation of Session Initiation Protocol (SIP). This library provides an interface to initiate and control SIP based sessions.

Project homepage for isdn2h323, a Linux-based ISDN to H.323 gateway.

SIP based VoIP phone. It supports Presence and Instant Messaging.

SIP based web phone. Supported audio codecs are G711, LPC10-15, GSM, and SPEEX.

SIP based phone. Supports video, communication encryption and push-to-talk.

Mini-HOWTO for NAT & H.323 - configuring Linux to support H.323 while using network address translation.

Program that allows two parties to hold a secure voice conversation using Linux and TCP/IP.

Free H.323 gatekeeper (GPL) based on OpenH323.

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.

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

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).

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

Peer to peer voice service. Users may call landlines and cellphones for a fee; users may call each other for free. Source code is not available.

VOCAL is an open source project dealing with VoIP. It includes a SIP based Redirect Server, Feature Server, Provisioning Server and Marshal Proxy.