I don't stand up so much as in get confrontational but I usually say "I stutter" very matter of factly and let the other person deal with the awkwardness they created. Very rarely, through my job I've had the odd person be a bit mean about it. But it gets me going. I usually don't start an issue about stuttering, but if they're rude, I will kind of passive aggressively extend the conversation as long as I can and try to annoy them into hanging up on me. I'm a bit of a troll and I like some excitement in life.