I don't have a job where I'm on the phone all the time. But if I did or was starting one, then I'd practice on the phone with some of my friends. I still stutter a lot with friends or close family so I'd be perfect practice. Then I'd move to people I don't know so well. Maybe a friend of a friend. And then practice with them. The point is progressive overload. Start small and easy and ramp up the difficulty. Get your reps in. The more you practice, the easier talking on the phone becomes. You'll get less blocks or they'll be less prolonged. I'd also have script that you can read off of. Like think of things you'd normally have to say in that job and use them in the practice phone calls.