Don't pretend to know something that you don't. Summarize for the subject in your own words some of his main points. Make the call when you are almost finished with the story. Use it as a second interview. Ask about areas you did not understand, or about areas that will be a part of the story but were not covered well during the original interview.
Tape record the conversation if time permits and the story demands.
If you decide to use a recorder, ask permission of the subject. Place it off to the side, but where it can be seen. Make sure it is in good working order with good batteries. Use it as a backup to your regular note taking. If unsuccessful, make sure you and the subject understand the ground rules.