A few ideas: first explain in text that you stutter and you prefer to doing business through text for that reason, but you can speak by phone if they like. Just get that out there so when you don’t talk it’s already out there and you aren’t trying to hide the stuttering. If you are in a position where you are negotiating you may be at too much of a disadvantage and you might need to insist on negotiation by text and the ball would be in his court. You can practice some likely scenarios by yourself or with a friend. You could have pat phrases and sentences prepared that you might say. Mostly the more you do these calls the better. It’s scary but you can do it.