asterisk users - Jun 2005 - connect to SIP trunk getting unable to create/find channel

I have a SIP trunk coming into my asterisk box.
the error is Unable to create/find channel.

How do I define that a DID number is handled in a certain manner???

Asterisk cannot find how to handle an incoming call from a number.

I have in sip.conf my definition to the PBX. That seems good as I can 
call out.
In that sip.conf I have a context=nortel. I thought that would take any 
calls coming
across and goto my nortel context in extensions.conf - doesnt seem to be 
doing that.

How do I tell asterisks to accept a DID number coming across a SIP trunk???

Thanks so much.

Jerry