Is either "in a call" or "on a call" incorrect usage when referring to someone attending a phone (possibly conference) call? If not, what's appropriate usage for both?
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" because it sounds similar to "in a meeting". You wouldn't ask someone having technical problems "are you on yet?" though. (This is a bit confusing, because we talk about logging on, but once someone has logged on, they are usually described as "in".)
"On" is more often used to describe someone who is in the process of taking part in a call more generally. "Will you be on that call this afternoon?". But you could equally say "in" here.