Why do men (or people in general) call someone and hang up after the person said hello?

Recently I got a call from an unknown number. The caller did not say a word and hung up a few seconds after I picked up the phone. I later found out it was a fellow student who must have found my phone number in the whatsapp group of a course at university we both were attending. I talked with him a few times but only consider him a loose acqaintance.

Why did he do that?

Why do men (or people in general) call someone and hang up after the person said hello?
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