How can I record both sides of a phone conversation via iPhone? (see details)

I am interviewing an 80 year old gentleman over the phone so he can relay very important historical information which I will transcribe. Both he and I wish to have the conversation recorded since he is too old to type it out and I cannot type fast enough to keep up. (Thus this is a LEGAL use of recording)

What is the best way to do this? I could use a land line too but prefer the iPhone since it has long distance.

Any help deeply appreciated.