Wouldn’t the rabbi have to be the source that revealed this phone call??? If so, my take is that the rabbi didn’t take the bribe and revealed it because he doesn’t like Biden or the Dem candidate.
White House calls are rarely confidential. It's on the agenda.
The Chief of Staff (Ron Klain), who ever's pulling Biden's strings (i.e., the person who put the rabbi's name on the list to be called -- someone at the DNC?), many of their support staff and assistants, the White House rumor grapevine, plus who knows how many people the rabbi's end knew -- all are possibilities.