I have been stuttering since I can remember. That didn’t stop me from doing many great things in life though such as a military career, a BS and a MS degree. I’ve been ridiculed and bullied because of it. I realize you work in customer service part of your time at work. I just brush it off my shoulder. That wasn’t always the case though. My speech therapist had me read a lot loud when I was a child. Today I’m a really good at reading. I remember listening to other children read out loud, and they weren’t very good at it. The old saying is, “practice makes perfect”. Anyways, I also used to beat up other kids or make fun of them for their own faults. It’s not the best answer but kids can be mean. Unfortunately some adults can’t do any better. From one stutterer to another there’s a few things you can do besides ignoring it and allowing it to aggravate you later. You could make something up like telling the person on the other side of the phone to excuse your self because tired due to working 16 hours a day without overtime since you’re salaried employee, or you had a seizure less than 12 hours before, or you have brain damage due to anxiety. This may sound ridiculous but it sometimes makes people feel a little guilty for being an asshole.