sipX ECS (Enterprise Communications Server) is an open source voice over IP-PBX best suited for large scale deployments.

In contrast to its main open source competitor Asterisk, sipX does not
play the role of a back-to-back user agent. This approach led to a modular and highly scalable system. All major components of sipX are implemented as servers and do not necessarily have to reside on a single
machine only.

sipX includes many features of a traditional private branch exchange (PBX) like voice mail, interactive voice response systems, auto attendants and the like. Furthermore it integrates with Exchange and Active Directory environments.

Eyepea selected sipX as its architecture makes it much more appropriate than Asterisk for large deployments where scalability and redundancy is key.

SIPfoundry + Community