If I called the ER about a patient of whom a doctor is taking care, should I say, 'may I speak to the doctor on the patient name John? I have a result on this patient?" or should I use for, "may i speak to the doctor for the patient name John? I have a result for this patient?" Thank you.
You are looking for a possessive like, „the doctor‘s patient“, but you cannot say „the doctor on the patient.“ That’s not grammatically possible. Something like „The doctor for the patient“ might be possible in some carefully drawn narrow sentences, like „For this patient, we have assigned Dr. XYZ,“ but otherwise this preposition will not work either. What you need to use is an entirely different sentence structure, like „May I speak with the doctor taking care of (or caring for) this patient.“