A call handling plan is a powerful tool that allows you to determine how calls to the mailbox are routed based on the day of week and the time of day, as well as by the call condition (busy, ring no answer, etc.). The Call Handling Plans page allows you to assign a call handling plan to the mailbox, to remove or disable a template already assigned.
Modifying call handling should be done with care, as difficult-to-diagnose problems can easily be introduced in this area.
The figure below shows the Call Handling page for a user mailbox, which is assigned the standard call handling template by default.
Call handling templates are configured globally here.