#9816 will get you a call holding on trunk 16.
If trunk 16 is loudspeaker paging, programming it as a paging trunk will let you use #9816 for paging.
If trunk 816 is in a pool, you can assign pool dial out code access to the extension, and you would dial 70, or 890, or whatever pool 816 is in.
Maybe we need to know why you need to grab just 816?