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.
The phrase "on a call" can also mean making a visit, usually an official one.
The terminology may have lost some meaning of late, but previously "in a call", in a multiperson office situation, meant that the person was physically in an office with several other people, talking via speakerphone with some remote party (presumably either a superior or some significant customer).
"On a call", on the other hand, would imply that the party of which you speak is is out of the office, visiting a customer location (or, for a doctor, visiting a patient).
(And, of course, "on call" means the individual is standing ready to fulfill some role, should she be called to do so. In this case physical location is unspecified and unimportant, so long as the person can be reached by phone, pager, etc.)
If you are in a call then when you stop out of the call? Clearly you are on the phone and not in the phone. Off the phone makes sense but out of the phone makes no sense. I vote for on a call and in a meeting because off the call and out of the meeting make more sense.
..if it refers to a 'phone call', the correct form is "on a call" as in "on the phone".
you can't be "in a phone call" for the same reason you can't be "IN" the phone.
"in a call, is street English!
"Call" can be a verb, noun, or adjective. Therefore you can say both, depending on what you mean. "Game" is one good example, if you're playing and someone called you, you can say, "I'm in a game" you could also say, "I'm on a game" It depends on the situation and what game are you playing. Same applies to "In a call" or "On a call". You can't be INSIDE the phone, if you're referring to the phone, that's a NOUN, but, if it will be a conversation, that'll be a VERB, or an ADVERB, in that case you now can say, "I'm in a phone call"