I experienced the same. Previous call centre employee here who was confident on the phone for months. Didnt stutter at all, I actually spent my lunch break taking calls.... then all of a sudden it progressively got worse to the point where I went to work one morning and butchered a call so hard I just quit the job. What contributed to all of this was definitely that I wasn't enjoying the job and having fun any more. You really need to spend your energy having fun on the job. Do your workmates go out for drinks after work? Do you do things when you're off the phone? I just felt pressured from the job and then it all crumbled in. I really think I could go back to a call centr job one day. It's so cliche and I hate hearing it but it always helped to remind myself that the people on the other end of the line aren't even people to me. They were just voices really. Just thinking about it now, it's just all a memory. Sorry to get all philosophical and all that but it's really just like that.