Is Asterisk a full fledged SIP Proxy Server?
Well. Not really. It’s a SIP client, connecting to other SIP proxies. It’s also a very good SIP Server, letting SIP clients connect to Asterisk to receive and set up calls, transfer calls, participate in conferences and much more. As of conforming to the SIP RFC, Asterisk does not support SIP over TCP or SIP over SSL/TCP yet. See Asterisk SIP channels and Asterisk SIP not-proxy for more information.