http://www.ibm.com/developerworks/wireless/library/wi-sip.html
In this article , author says 'A SIP proxy server typically handles registrations, implementing call-routing policies, and performing authentication and authorization. ’
If Sip proxy implements Call routing, then Do i need to have a PBX such as asterisk if all i have to route calls b/w sip User Agents ??
What differentiates PBX from SIP proxy,Except that PBX can understand other protocols obviously.