Craig, this is easy to do....I’m sure there are other ways but this one works for me.....

Create a Calling Search Space, Call it something like Internal add only partitions that can NOT call off net. Worst cases add nothing to it. (Not recommended)

Now go to the Directory Number Configuration page for the phone in question, in the Call Forward and Pickup Settings add the Internal CSS to the CFA/CFB/CFNA accordingly. Update phone.

Remember if there is a need for any of the CFA/CFB and or CFNA need to go off net you may have to find another way or create a better CSS that will fit your need.

Good luck

Rob