I use the phrase "call handler" but it doesn't necessarily have to be done in Unity.
CUCM 6.1 and Unity 5.01
The DN is 7859.
During the hours of 7am to 5pm, if you dial 7859 it should ring a hunt group.
During the hours of 5pm to 7am, if you dial 7859 it should ring an outside answering service, 9-xxx-xxxx.
First part is easy, I have that working in a Unity call handler. The second part is causing trouble as the Unity closed greeting does not have an option to transfer to an outside number, only another subscriber.