What is the difference of being in a meeting and being on a meeting?

hope that answers your question x "In a meeting" is the generally used phrase. "He is in a meeting". "I'll call you back. Right now, I'm in a meeting".

"...on a meeting" would only be used in conjunction with some other phrase, such as "sit in". "He sat in on the meeting". In other words, he is not normally part of those meetings, but he went in to observe for a bit. Another would be "Listen in". Other than that type of phrase (there are others) it will always be "in a meeting".