asterisk users - Jun 2009 - How to set the II DIgits?

I need to set the II digits for some outgoing calls originating with  
asterisk, but the documentation seems to show that all the various  
ANI2 variables are read-only.  So how do I set them?

(Yes, we have Feature Group D trunks and allowed to set them and  
regularly do with our C.O. switch.  The interface between that switch  
and asterisk is via ISDN spans.)

Thanks...

Jim Gottlieb
San Diego, California