One cannot always distinguish memories laid down while dreaming from those acquired while awake. It's quite a common experience. Almost everyone must have heard someone exclaim "I could have sworn that xxxxx! How strange! I must have dreamt it." And I expect most readers will be able to recall having had this kind of mildly disorienting experience themselves.

Something else that is very tricky about dreams, is that fragments of a dream may pop into one's head, triggered by some sensation or mental activity, long after the dream. Again, most of us will be able to recall thinking "I just remembered a dream I had" -- and usually adding "last night". But of course if we cannot always reliably distinguish dream memories from waking experience, then we cannot regard our recollections of *when* we dreamt something as necessarily accurate.

You can probably see where this is going. It is quite reasonable (null hypothesis, even!) to imagine that a person may occasionally dream of a long lost friend. And it is, if anything, even more reasonable to imagine that any memory traces of such dreams would be activated by picking up a ringing phone and hearing that friend's voice ...

And "Voila!" -- there you are gasping,
"Heck, it's Harry! Harry, this is amazing, I dreamt you'd call."