Excuse yourself momentarily
If you feel the telltale rumbling of gas if your belly about to erupt, excuse yourself to the restroom if you can. If for some reason you can’t get to a bathroom (like, if you’re on a hike or something else outside), pretend you have an urgent call and excuse yourself to take it a few yards away. Afterward, apologize profusely for taking the call and explain that your sister/great-aunt/boss never ever calls you unless it’s an emergency, and that it ended up being a butt-dial.