When people started calling my personal mobile number with questions about a voluntary organisation I'm involved with, I was confused: we weren't sharing that number. It turns out that Google had decided to take the number I used to verify my identity for Google Business some years prior and start putting it in Google Search results. WTF, Google?
Get their phone number published on the open internet.
If you wanted to do that, you could just edit it into a business listing directly, or even create one. If it’s from the “was this a business” prompt, I expect it takes a bunch of people to say yes, not just one. (And if you get that prompt, Google associates your number with a business already.)
I expect the algorithm is a) not publicly published, and b) changes periodically - like to just require one click when a particular department needs to “make their numbers” by the end of the quarter.
I challenge you to find any “free and easy, one click” business listing that gets 0.1% of the visibility of a Google info business phone number.