Using "will" in a sentence vs not using "will"?

Question

I'm an ESL teacher. A student asked me a question regarding this sentence. ***"Don't phone Ann now. She'll be busy."***The student asked me why we have to use "will" in the sentence. Why shouldn't the sentence just be:***"Don't phone Ann now. She's busy."***I told her sentence one is predicting what is and the second is reporting what is, but I feel like I short changed my student. Perhaps there's a more detailed explanation someone might have?

Answer 1

Your explanation to your student seems correct to me. "Will" can be used for "almost certainly" versus "is," which would have sounded like Ann is, as a matter of fact, busy.

Answer 2

Your correct advice to use "will" may have a simpler reason.

The student's call will take place in the future, even if only a few minutes or an hour. The use of "will" makes it clear that the student will call and Ann will be busy in the future.

Using "is," on the other hand, would suggest that Ann is busy right now at the time the two people are discussing when to call.