RLyon,
The feature code for Outgoing is, by default 8, and the feature code for ARS is, by default 92000. You would still need something in the Outgoing Extension of the phone as there is nothing a user can do to put something in there.
What most technicians do is set the Outgoing Extension to ARS, default 92000, and then set the Class of Service on the phone to ARS Only. Then they would setup the Account Code Type to be Forced All Calls.
You would have to create an account code for them to use for this, but that is pretty simple.
Thanks,
TE