Not a call centre, but I work for a mental health service where we assess and treat (provide CBT) clients over the phone. Can get very demanding, physically and mentally. I often tell my clients I stammer, who relaxes me a little bit. It’s the time pressure which I find most Impacting, since I know I have to speed up, yet when I speed up I end up stammering more and the cycle continues really. Just take your time, allow yourself to stammer, and see it is a great learning experience for yourself as a person who stammers!