Don't quit! I also stutter and I took the same challenge and I have been working at a call center for the past 1 month, yes the most difficult part is the call opening but after that it's nothing. No doubt it's difficult to start your day when you are rowing against the tide and then maintain all the metrics like average handling time and customer satisfaction etc but give yourself a chance. Rember there is only a human being like us sitting across the other side of the call. I would only request you to keep trying and if it didn't work let them terminate you but Don't Quit!