1. All my call back are gone, where are they?
If you look at "Call-back" on the menu on the left, often you can't find them, because you've searched for them:
If this is not the case, it can be that someone have changed it trough a filterview or warehouse. This is only something the admin users can do.
2. Why did a call back from one phoner turn up at another phoner?
Private call backs can come to other phoners if the private call backs should expire, which is defined under the campaign settings (Settings --> Campaigns --> Edit --> Leads --> Private call-backs expire) and the original phoner has not called the customer before it expired.
The phoner who received the private call-back from another phoner could've searched for the lead and opened it. This can be seen