when i was tapering off pred my gi wanted me to check in every week via phone. if i didnt get her when i called she would call me back same day, later on. also when i have had major problems she always called me back.
For other types of drs and specialist (for example when I was preg) I would usually just speak to a nurse or have nurse relay info. This worked out fine at the time.
If you hang up and have more questions, then just call them back. I know its easy to feel intimidated or like you are being a pain, but if you are nice about it and appreciative to the staff, they shouldnt mind.