asterisk users - Dec 2005 - SPA-3K FXO: Incoming and outgoing calls in different contexts?

I'm working to get my head around some of the conceptual underpinnings 
of SIP channels.

In a couple of recent discussions, Kevin and others have noted that the 
notion of separate "user" and "peer" behavior from a SIP
partner (for
want of a better word) is not really germane, as it is in the world, 
say, of IAX.

So I'm trying to apply that to the FXO port on my SPA-3000 with respect 
to its ability to both originate and terminate calls to/from the PSTN?

I wonder what the cheeses would recommend.  I want to be able to 
differentiate behavior based on the originate/terminate distinction, 
which is easy when one distinguishes between the user and peer entities 
in an iax.conf file.

How would I do that if, as recommended, I have a single entry in the 
sip.conf file for the channel, defined as a "peer?"

Thanks in advance for any good ideas out there, and a happy new year to 
all. . .

B.