Haven't had many such calls in May. I keep telling my clients not to accept them as most times they mention Windows and anyone using a computer will correlate that to Microsoft. It is somewhat illegal for them to mention Microsoft due to copyright/trademark issues [our brain seems to do that assumption automatically]. Have never know Microsoft to call a computer owner 'out of the blue', the user had to initiate the call to them.
As for the date of expiration, the Insider Preview version of Win10 does carry a date, mine are showing December this year.