At the time when a callback is scheduled to occur, outbound dialing inserts the callback at the top of its dialing queue. If the campaign is turned off, leaving scheduled callbacks to occur at various times, the campaign dials the call that came due last as the first when it is restarted.
Example:
A campaign is turned off, with outstanding callbacks scheduled for 8 AM and 10 AM the next day. When the campaign is turned on at noon the following day, both callbacks are due. The one that was due at 10 AM is called first, followed by the one that was due at 8 AM. This happens because the 10 AM call was at the top of the dialing queue.
Note: This behavior only applies to non-preview rule scheduled callbacks and recalls. Agent-scheduled callbacks and preview rule-scheduled callbacks always place the callback call when it comes due, regardless of whether the campaign is on or off.