Here's how you know an email is a BEC.
Just look at the text. The logic falls on itself. Why would the chairman of a company email you about an emergency, and then ask you to give them your number? They already have your attention via email.
Or look at the sender address. In this case, it's a Gmail address with a name that's very different from the chairman's actual name.
Further, everyone knows that email is not a platform for immediate communication--Slack and Teams fit that bill.
So if you get an email like this, what should you do? Follow the signs. Think through the logic. Ask IT for help.
Or, better yet, find a solution that just blocks it automatically.