callbacks are meant to be made manually, not automatically. the agent will receive a callback notice in their session. that being said, look at the status of the lead when you create the callback. it is assigned a status of "callback hold". when the time arrives for the call to happen, that status (if i recall) changes to callback. so you set your campaign to have the status you want to dial (which would be the one it changes to after the time arrives). experiment.
but i don't suggest it: the agent is supposed to make that call manually to avoid dropping the call. after all, if the system generates the call automatically ... and then the agent gets another call from the outbound calling system .. your agent will be busy when the callback prospect answers and the call will be dropped. not a good result from a "likely high sale probability" callback. those are golden, handle them manually.