First off, were not told about that aspect of the job to begin with? Kinda strange that you landed that job in the first place. But you could also look at it as a thing to overcome. You can quit and run away from having to speak on the phone. Or you can view it as a challenge to overcome. If you somehow figure out a way to get over this fear, I can guarantee your speech will improve if you're patient. The only way my speech improved was because I searched for ways to make myself uncomfortable and just got through the situations anyway. I still hate speaking on the phone, but a lot of the time, my stutter isn't even noticeable now.