There's not really any calibration involved, either the buttons are pressed (and the electronic switch is closed) or they aren't.
If you turn off the phone and restart it and the problem is still there, you probably have some hardware issues causing the switches to stick in the closed position. I'm assuming that there is still the normal time delay between the voice control and slide to turn off features are activated.
Do the buttons still "click" when you press them? If not, the contacts may have shifted.
In addition, moisture damage could also cause the same symptoms - in fact, I'd say that corrosion from liquid exposure or debris between the switch contacts is more likely to be the source of the trouble than impact damage. From your description, the case sounds like it's been breached and is no longer providing much defense from dirt or pocket lint.