Ben Carter comment:

I think you may have missed the point slightly on this feature.
The point of sending an acknowledgement is to let other people, who have been notified of the problem, know that someone has acknowledged it (i.e. they have picked it up) and therefore others don't need to worry about it.
See comment from 16th March 2013:
“On the Media Types CONFIGURATION OF ACTIONS screen there needs to be
an acknowledgement message checkbox and configuration as was done for
recovery message.”
As in, on recovery, you have the option:
"Notify all who received any messages regarding the problem before"
but on acknowledgement you only have:
"Notify all who left acknowledgement and comments"
Imagine the scenario, you notify 5 admins, then in a further step you notify 5 more in escalation, then maybe 2 managers. If someone from any of that acknowledges the problem, you should be able to send that acknowledgement to everyone so they all know it's been picked up.
This is impossible with the current options.
What is needed is the option: "Notify all who received any messages regarding the problem before" to be available for acknowledge operations.