I am so sorry, that is absolutely disgusting. Can you try calling again and start slowly, calmly and assertively with “I have a stammer. Please don’t hang up on me and let me finish what I have to say.”. Write it down first and read from it if it helps. If it happens again please email and complain in writing. That’s absolutely abhorrent behaviour and quite frankly I wouldn’t tolerate it. I stutter significantly more on the phone than anything else. I have to admit I’ve little tolerance now for people that don’t have the capacity to let me finish my sentence after years of this sort of thing. We shouldnt have to ask others to make phone calls for us. It’s the person on the other end of the phone that needs to change. We make room for and accommodate all other disabilities, therefore I do not know why we cannot do the same for people who stutter.