(1) Windows 10 smartphones currently make cellular voice calls so that feature is already in the mobile version of Windows 10.

(2) There's no reason why Samsung or one of the Chinese manufacturers couldn't make a Windows 10 phablet. Microsoft made Windows free on small-screen devices, so I assume it will continue to be free, or at worst, "zero paid".

[Microsoft had a Windows offering where OEMs paid $10 for the OS but got a $10 rebate for setting Bing as the default search engine. So technically, Windows wasn't free, but OEMs didn't pay anything to use it.]