I am not a teacher, but when I can't put a call through to somebody because they are seeing a client e.g. then I normally say "Mr XYZ is in a meeting at the moment..." So I would say "in a meeting/conference" when the meeting is happening now and I know that it is happening and it is happening here in the same building. Whereas I would use "at a meeting/conference" when the event is at a different place from my location. It would also imply that I know that the meeting is happening, but not necessarily at the moment in time when I say "He is at a meeting". For all I know they might be having a coffee break, but I would still say they are at a meeting/conference.
I hope that makes sense somehow.