How functional is the SIP server built into IOS? Does it allow clients to register/authenticate via local authentication on a smaller router then initiate outbound calls via, say an E1/FXO card? What about the client? Can you authenticate against a SIP proxy and then use that to initiate calls from one of the FXS ports? From what I've read, there seems to be no command in any current IOS to allow you to enter credentials into the sip user agent configuration context, so I'm guessing the answer is no.