This happened once when I was at work, I had to call another store to check on some stock for a customer and my coworker on the other end laughed/giggled whenever I stuttered. I called her out on it, let her know what's up and that I can contact HR if she has an issue with it. This was when I started working there, I wasn't going tolerate or sacrifice my comfort for someone else's. It never happened again. it's OK to put your foot down on situations you're not comfortable with. How you feel matters too if not more.