Facetime calls are encrypted, so there isn't really a straightforward way to "hack" the actual calls (well, maybe the NSA could). In fact, in this regard, FaceTime calls are probably MORE secure than conventional phone calls.
With respect to past calls, it is easy to see who you have called. This would require access to your iPhone, or I suppose they possibly subpoena Apple for the info. All you need to do to see your call history is open the FaceTime app and look at the contact list. It will show the last audio or video call you have had with each person. It will not show older call history, as far as I know.
I haven't used FaceTime in a while since my employer switch to Android phones, but you used to be able to delete your FaceTime call history by opening the app, then going to the RECENTS tab. Once there, click on EDIT and red circles appear beside your contacts. Click on each contact whose call history you want to delete, then click on DELETE. As I said, this is how it used to work, I'm not 100% certain it is still like this.
Good luck, hope everything works out for you.