asterisk users - Jan 2006 - SIP register vs SIP with a fixed IP

Hi,
 
Two questions for the gurus out here:
 
1) I recently asked, for a number of reasons, to have my provider change his
way of doing SIP wth me: instead of registering with his server,  I know
simply send my stuff to his IP without registration.
 
I have always had two test numbers: one IAX and one SIP.  When I get a call
on either of those numbers, the call is bridged and send back over SIP to my
land line.
 
Since I made the switch to fixed IP (although it may have occured before
without my knowledge) I get the following error ONLY when the call comes in
via SIP

"Got SIP response 488 "Not Acceptable Here" back from
111.111.111.111", this
being the IP I am sending my calls to.
 
When it comes through IAX, the call is bridged without any problem.
 
What could be the cause of this problem?
 
2) The way my contexts were setup before is that when a call came in through
555-555-5555, it landed in context_a.  when it came in through 555-666-6666,
it landed in context_b.  This way I could have a different dial plan per
customer (since I resell VoIP services).  This was easy before I was using
SIP registration.
 
Now that I am using a fixed IP with no registration, how do I switch the
call to a different context based on the number called?
 
Mike
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