That's because the realization of someone or something is listening to you makes you stutter , that's why you don't stutter alone ,even though, when you talking to someone on the phone you still stutter even though you're alone , but why ?, cuz you know the person on the phone is listening to you , and it doesn't have to be a person ,literally anything listens to you and can interact depends on things you say, like when talk to an AI , Siri , or anything , you'll (probably) Stutter . Hope that helps !