Early dial asterisk conf

I have been using freepbx but decided to learn asterisk and hard-code everything.
I have early dial set in my grandstream phones.
This works fine on freepbx.
I have searched the net for several hours trying to figure-out how to implement early dial in the asterisk dial plan.

Early dial requires a 484 response back to the phone. When I press the first digit I get the busy tone. The early dial is not working.

How do I configure the support for early dial in the Asterisk dial plan?