19. The two are used interchangeably as to be in or on a call is a fairly new expression, with some very slight contextual differences. "In" is more often used to describe whether someone has successfully connected (especially in conference calling): "Are you in yet?". However, it is sometimes used more generally "I'm sorry, he is in a call ... More @Wikipedia
Hover over any link to get a description of the article. Please note that search keywords are sometimes hidden within the full article and don't appear in the description or title.