I worked in Tech support at Microsoft for 7 months and English is not my native language but all of our customers were English speakers (EMEA region) so you can imagine the horror, I was so nervous at the start to make calls with the customers and still do tbh :P, but I realized that your stutter it doesn't matter for the customer as long as you know what you're doing and call help him ASAP. it's hard to deal but your knowledge in your field can make it not visible. I got a 4x better job offer and left the tech support for good, but hey we gonna use that damn phone everyday.