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

Videoconferencing application to make audio and video calls to remote users. Supports both SIP and H.323.

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.

SIP based VoIP user agent. It supports Presence and Instant Messaging.

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

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

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 a masquerading firewall (NAT).