For me, since as long as I can talk, I've had repetitions (b-b-b-) and blocks (not being able to get the sound out of your mouth), nowadays, it's mostly blocks and having to restructure the sentence and choose synonyms to avoid repetitions. I probably only get repetitions every few sentences. It happens whoever I am talking to but if I am stressed or tired, it becomes more severe. It also becomes worse when talking to people who draw attention to the problem or if I anticipate stuttering. You might already have a stuttering problem that you mostly know how to circumvent or you might usually speak fluently but be unable to when you have the phone anxiety. I wouldn't be able to say.