Is it advisable to calibrate the battery on a iPhone?
According to this Apple Support Communities user tip:


  So what are the "rules" for charging? The most basic one is charge
  whenever you want to, for a long as you want to. There's no reason to
  let the device drain completely before charging (in fact, it's a bad
  idea to do that on a regular basis), and there's no need to wait until
  it reaches 100% before removing it from the power source.  You can
  charge when it's at 40% and disconnect when it reaches 80%, or any
  other values, without hurting the phone. And you don't have to turn it
  off to charge it; in fact, you shouldn't.


There is no reason to calibrate the battery.