SIP Express Router (SER) is an industrial-strength, free VoIP server based
on the session initiation protocol (SIP RFC2543-bis).  It is engineered to
power IP telephony infrastructures up to large scale. The server keeps
track of users, sets up VoIP sessions, relays instant messages and creates
space for new plug-in applications. Its proven interoperability guarantees
seamless integration with components from other vendors, eliminating the
risk of a single-vendor trap. It has successfully participated in various
interoperability tests in which it worked with the products of other
leading SIP vendors.