In a meeting or at a meeting
In AE, we usually say “in a/the meeting.” However, much depends on context.
For example, if you are in the meeting space and are participating in the meeting, we use “in.”
In certain situations, you might use “at,” but that indicates that perhaps the meeting is on break and you are outside of the actual meeting space.
For example, let’s say that I attend a conference for PR professionals and I call my husband.
I might tell him: Hey honey, I’m at the conference.This means that I am at the venue where the conference is taking place. I am not, at this moment, actively participating in the conference.
Lets further say that while attending the conference, I meet with several other PR practitioners. During this meeting, my husband calls me on my cell phone. I might answer it* and say: I can’t talk right now. I’m in a meeting. In other words, I am actively participating in the proceedings.
* I would never, unless I knew it was an emergency, answer my cell phone during a professional meeting. I almost always have it turned off.