Callback
The application is intended to call back the subscriber using a particular gateway.
Call - a call, which will be denied and the system will make a call-back.
Gateway - gateway to use for callback. The system will make a call in accordance with the priorities of the directions if the gateway is not specified.
Number - a number to make a callback to. It could be either the caller number (choose a variable @Call #Caller Number) or any other number.
