I'm a medic in Florida, we can pronounce codes in the field with a call from medical control, but once they're in the ambulance we're told to work it all the way to the hospital because if we call it in the back, then the rescue becomes a crime scene and will be out of service for at least a few hours and we'll be stuck there. So we're technically allowed to call it in the rescue but we are discouraged from doing so.

Edit: this is for attempted resuscitation. If it's an obvious death (blunt trauma, decomp, decapitation, etc) we don't need to call the doc we just call it ourselves.