Hi Shibashish,
I am afraid there is no simple standard solution, unless you have CTI attached which will trigger the call at proper time.
We have solved the same task, that you have, with some custom coding and developed kind of ABAP dialer, which utilize the polling SAM mechanism. The callback reminder is then based on dedicated Call List and the dialer just select proper item with a Contact document attached to the Call List item.
If you create a custom event in the repository, it will not help you, I think. You will still need coding, which will actually trigger that event. The event is an anchor, to which you can then attach rule policy evaluation trigger.
Regards,
Dawood.