I think their point is in physical space, dozens to maybe thousands of people (if there's a lot of people around you, I guess?) can easily interrupt you at any given moment. With phones and things like Slack, hypothetically anyone near a phone can interrupt you if you're near your phone. Which people usually keep near them.

I would say depending on how bad someone has it they could get 1 to 3 spam calls a day, I assume if someone was getting consistently more than that they'd use a screener to lower it. That's a significant amount.