sipX ECS (Enterprise Communications Server) is an opensource IP telephony systems including many features of traditional PBX such as voice mail, interactive voice response systems, vacation responder, and many others. Its main feature is a software implementation of the Session Initiation Protocol (SIP), which makes it a communication system based on IP (IP PBX). sipX is available under the GNU Lesser General Public License (LGPL).

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Similar in functionality to Asterisk, a popular open source PBX system, but the design philosophy of sipX differs from that in many respects. The system was designed around a router-switch SIP. Unlike its main rival open source Asterisk PBX and other commercial offers that use SIP only as a transport protocol, SipX don’t use a back-to-back user agent paradigm. This approach has led to a modular and highly scalable design. All major components are implemented as sipX services to run server side and need not reside on a single machine.
Features
All reports of calls are managed using SIP natively (vs. gatewaying SIP or some other signaling protocol, such as the Asterisk PBX).

SipX components handle the original alert, but once a call is initiated packets (voice traffic) are sent directly between the endpoints involved. This allows most of the components of sipX be transparent in the media and its encoding, and for example to-SIP videophones to communicate without increasing the load on the sipX system.

The system architecture is client-server and not monolithic, the various components of sipX (proxy, media server, etc.) can communicate with each other via the SIP protocol has been installed on different hosts (or replaced with other components SIP).

The system administration interface is web-based (vs. line interface).

sipX adheres to the SIP philosophy consisting in the implementation of many features directly through the endpoints (phones, gateways, voicemail systems), rather than by the main server components (proxy). This improves scalability, but many features depend on support in the endpoint of the telephone system.

It integrates with Echange and ActiveDirectory environment.