iPhone.
I want to hear a vm I left for someone.
Ask the person for their voicemail access code.
Impossible
No. I am sure you can figure out how something like that can be abused.
Sigh.
S.O.L. Sorry that is a one way.
Usually voicemail systems have a way to review and re-record your message if needed. However, this must be done at the same time as leaving the original message. You cannot call back and then edit the message.