asterisk users - Nov 2003 - Cisco router/SIP gateway registration

Is there a way to have a Cisco SIP gateway register with Asterisk?

The current setup just drops calls into the sip.conf default context 
which works fine but has some security risks since anyone who can 
install XTEN and has access to my LAN can then use this context to drop 
calls in....

I'd like to be able to get inbound calls from the cisco in a from_gw 
context, then I can just set the default context to a simple Congestion 
dialplan...

Thanks!