More and more companies count on Asterisk to provide their most mission-critical application: dial tone. Asterisk has what you'd expect from an enterprise-worthy VoIP system, and integrates with other tools to allow for clustering, failover, IPv6 support, and a range of other functions. Asterisk operates as a back-to-back user agent, managing all aspects of a VoIP call, where some of its contenders deal only with VoIP signaling. Asterisk is not without rivals, notably OpenSER (recently renamed Kamailio), FreeSWITCH, and CallWeaver -- but it's still our champ. FULL REPORT ...