Changing busy message depending on type of busy


I am trying to make it so when a person enables DND the phone system doesnt say they are on the phone, instead it plays the person is unavailable message.

The reason for this is because we run a large freepbx site of near 600 phones and our receptionists need to know when a person is on DND or if they are on the phone.

I know we could use the likes of iSymphony (which we are) to check if they are on DND however this process is alot slower than simply dialing the number.

Does anyone know of a way to do this?