The weirdest part about it is that you're comparing one specific dream to one specific event, but really your brain knows your cousin, knows he calls from time to time, probably already heard about important stuff going on in his life. so it's not like predicting something impossible, it's more your unconscious putting together clues you already had and creating a plausible scenario. it happens way more often than we expect because we're surrounded by information we don't consciously process.
the real problem is when you start putting too much weight on these coincidences and change your behavior because of them. like, you start waiting for "warnings" from your subconscious, you wake up from the dream and you're already anticipating the phone call, and then when it comes it seems prophetic when really you were just more paying attention to it happening. your brain loves confirming what it thinks it already knows.
the thing is: accept that it was interesting, but don't build expectations around it. it's not worth spending time monitoring your dreams looking for patterns, because you're always gonna find some that "match" just because of how many dreams we have in a week. coincidences that make noise are always more memorable than the thousand other things that didn't lead to anything.