It gives the other person a chance to tell you if there is an emergency / tell you the urgency of their request, and also lets them know that you know they want to talk to you.
That works too. Both Apple and Android have built-in support for do-not-disturb policies that will let someone through if they call twice in a row (defined as "within 3 minutes" on Apple and "within 15 minutes" on Android, it seems).
