I'm sorry, but huh?????
So if you are in town A, and a resident in town B directly dials you to take them to the hospital, can you do it?
The simple answer is, you can take any call in any town you want, as long as it is a direct request. Paid commercial transport companies do this all the time (lights and sirens to pick up people in nursing homes, etc). Whether or not an agency is volunteer has nothing to do with it.
Some agencies will, as a courtesy, notify the town you are responding you that you are taking a job, so they don't wonder why you are going lights and sirens through their town to a job in their town.