Echoing the others - depends. Often both.
Definitely in the pre interview stage. I personally got quite a few interviews this way. I have a friend who did not get an interview at a particular top ent program she liked. Her mentor made a call, they gave her an interview, and she matched there. She was a stellar applicant already so was already probably on the short list but for some reason didn’t make the final 30-40 people who got interviewed. So the pre interview call can be very helpful. I also used it to get interviews at a couple distant geographic programs that I was sure would probably not interview me otherwise.
The post interview contact is less frequent and often initiated by the other program. This is more to get off the record real comments from people you know and respect and will have to look you in the eye in the near future if they don’t give you the truth. Post interview calls seem less relevant since you already know the applicant a bit and have your own impression. If someone was a real tool, I don’t think anyone is going to change that impression.
