The phone or radios and anything where there's a feedback of hearing myself speak drastically increases my stutter. The best way I can treat myself is firstly to do it all the time so I don't get too anxious about which will happen as part of your job Secondly. I have a very well Rehearsed "hi how are you?" that I know I can say confidently but you have to wait for them to respond so they know you actually want to know how they are. I find this catches alot of off guard and makes them feel like a person rather then a means to an end on the phone call. The trick is that it does the same thing in their opinion of you, and creates a much more face to face vibe and even if you do stutter a bit they tend to like you already so they're much more understanding. It's also just better business to operate that way.