They email, I'm currently going through verification but my status finally changed after 7 weeks to "In progress", hope that's good haha. If they can't find a number or contact for HR you will get an email for a specific HR contact. If they can't properly verify that the contact you shared is indeed HR they will likely ask for documentation on your part (contract, offer letter, paystubs, etc.).
Its pretty simple, yet since its a blackbox (No one knows what the process is really like behind the curtain) it definitely makes people anxious.
P.S. if there are any prior Booth admits who could confirm what the timeline was in your case I would appreciate it. I started the process in February but only heard from them until March. Haven't heard back since and my status changed today to "in progress", so I am wondering if they are almost done or just beginning haha.