Are you sure precommitment is a useful strategy here? Generally the use of precommitments is only worthwhile when the other actors behave in a rational manner (in the strictly economic sense), consider your precommitment credible, and are not willing to pay the cost of you following through on your precommitment.

While I'm in no position to comment on how rational your parents are, it's likely that the cost of you being upset with them is a price they're willing to pay for what they may conceptualize as "keeping you safe", "good parenting" or whatever their claimed good intentions were. As a result no amount of precommitment will let you win that situation, and we all know that rationalists should win.

The optimal solution is probably the one where your parents no longer feel that they should listen to your phone calls or use physical coercion in the first place. I couldn't say exactly how you go about achieving this without knowing more about your parents' intentions. However you should be able to figure out what their goal was and explain to them how they can achieve it without using force or eavesdropping on you.