There really isn't a good way to justify your actions. The phone wasn't yours and if it was a bother then moving it into her purse or a drawer and shutting the door would have been an effective and more acceptable alternative if you couldn't find the volume button. Recent calls you miss often don't show up after you turn a phone off and it may have been an important number she needed, but that's not even the most important point in this situation. The important point is that it isn't your phone to do what you want with. You may have had good intentions or you may have just been annoyed, yourself, and opted to just turn it off. It's really her situation to deal with. If ppl are annoyed then they'll have to confront her about it, it has nothing to do with you even if you are her friend. Since we're in the "Etiquette" category I would like to add that it is poor etiquette to have your phone on ring and not vibrate or silent while in the work place. Not professional. Office manager or not, friend or not, boss or not (etc) it's someone elses property and the appropriate course of action would to be to find the person or confront them when they return and if you are in a position of authority you can make their misconduct known afterwards. The interruption, if any, will better show the employee why it is the office has certain rules. I believe you take an unentitled amount of power upon yourself when you innapropriately "take matters into your own hands." All in all, you did cross the line and it'd be best if you humbled yourself for just a moment, long enough to tell her you understand where she is coming from and that you sincerely apologize for what you did. Do you really want to be that woman that sticks to her defenses even if she's wrong? Nobody really wants a friend like that. Good intentions or not.