Actually it is the physicians who are the volunteers since they are dedicating their time to answering your questions, getting the patients' permission, etc. They are the ones who are doing something they don't have to do and aren't getting paid for it. I could see how the physician could get volunteer hours for letting you shadow. Shadowing is clinical experience but not volunteer experience.
If you are asking what to classify it as on the AMCAS, I put it under "other." Many others put it under "Medical-Volunteering" (or something like that). Although technically not correct, I'm sure adcoms won't care.