When I first started my job, I started to get a lot of chest pain and discomfort in my chest, similar to what you describe. I work in a call centre too so I'm talking all day everyday. It isn't too bad, unless I'm in training for a new call stream, I get more nervous and doubt myself more which can cause me to stutter more. There are some words which I stutter on more regularly than others, such as limited, sitting, Saturday, September, illuminati, etc and so I will usually force myself through those words. When I'm on a call, we have a script to stick to and I can't often change the wording so if I stutter, i have to push on and Continue, which can be embarrassing at times.