The only way to accomplish that is to create another trunk group ( extension list and route calls out of that ) . I have not heard of anyone doing 3 groups , only 2 as it cuts down on your available PRI channels .
You should have 2 routes in your system now as we use them for 911 purposes
First trunk group has an assigned pretranslator and the 2nd trunk group has another pretranslator assigned for out bound caller id for 911 calls
1 May be the business main number and the 2nd one may be for caller id ( 10 digit number of the telephone )
This is a usual scenerio that we see a lot of . .