Not sure if I have all of the terminology right, but I would like to have Asterisk register with openser.  Then when a call comes in I would like to construct the sip uri using the Dialed number and the user location.  The final uri should be:

 

Sip:240744XXXX@25.25.25.25.  Where the IP address is the user location of the Asterisk box.

 

Any suggestions.