Sippy B2BUA is a RFC3261-compliant Session Initiation Protocol (SIP)
Back-to-back user agent (B2BUA) server software. 

The B2BUA is a SIP call controlling component. Unlike a SIP proxy, which only
maintains transaction state, the B2BUA maintains complete call state and
participates in all call requests. For this reason it can perform number of
functions that are not possible to implement using SIP proxy, such as for
example accurate call accounting, pre-paid rating and billing, fail over call
routing etc. Unlike PBX-type solutions such as Asterisk for example, the B2BUA
doesn't perform any media relaying or processing, therefore it doesn't
introduce any additional packet loss, delay or jitter into the media path.

Features:
 o Realtime calls control and call data records (CDRs) generation; 
 o Optional ability to use Sippy RTPproxy for media relaying; 
 o Optional ability to perform Cisco-compatible RADIUS AAA (Authentication,
   Authorization and Accounting); 
 o RFC3261 compliance; 
 o RFC3326 support; 
 o IPv6 support; 
 o Seamless compatibility with majority of popular SIP software and hardware
   on the market today;
 o Robustness and Resilience.

WWW: http://www.b2bua.org/