Looking at the trailer and the subject matter of the first part of Phantom Pain I would guess that Ishmael is a figment of Snake's imagination rather than a real person hence Keifer's voice. I know it's been suggested that Ishmael could be the medic but i'm not sure. If there's one thing you can say about Hideo is that if the medic's voice is Keifer then there will be a reason for it.