If you are accepted at A off waitlist and then accepted at B, B has to call A by EOB
https://www.aamc.org/download/364264/data/2014trafficrules.pdf
After May 15, any school that plans to make an acceptance offer to an applicant already known to have been accepted to or granted a deferral by another school or program for that entering class ensure that the other school or program is advised of this offer at the time that an offer of acceptance, either written or verbal, has been communicated to an applicant. This notification should be made immediately by telephone and email by the close of business on the same day. The communication should contain the applicants name and AAMC ID number, the program being offered (MD only, joint program) and the date through which the offer is valid. Schools and programs should communicate fully with each other with respect to anticipated late roster changes in order to minimize inter-school miscommunication and misunderstanding, as well as the possibility of unintended vacant positions in a school's first-year entering class.