How to change DND voice mail response in FreePBX?

I am trying to sort out how to change it so that when a user toggles DND, they get a response saying the user is unavailable (default answer, not a custom recording) instead of it saying they are on the phone.

I am in the process of displacing and Avaya system, and apparently that is how it responds, and they want the same behavior on the new FPBX system.

I am running the current stable Distro, so FPBX 2.11.x with Asterisk 11.4.x, and am using Astra SIP phones if that matters at all.

I already told them they could record a message to say anything they wanted, but they want the default behavior changed, so any ideas??

-Howard