The Mayo letter means nothing as to your status of being "ranked to match." If you are ranked within their top 12, you will receive a personal phone call. I don't know if more than one person makes the call, but I received my call from the assistant program director.
In regards to Iowa, if you are ranked within their top 6, you will receive a personal call(s) from Dr. Buckwalter.
I don't remember the exact dates of when I received these calls, but I remember Iowa was earlier than Mayo. There are also other programs that make personal calls, and some are as late as the Monday immediately prior to Match Day.