OK, let me re-staqte that.
I think I told it wrong.
When you enable VOICE OVER BUSY to an ATL Set, you have to use the BLUE pair of the next extension up.
So then you could use that to feed an amp.
And yes, test this with a butt set before going to any expense.
I am no where near my lab, and won't be for quite a while, or I'd test it.
But I'd be willing to bet it will work.