Stick with it. With a job like that, talking on the phone if it's a regular thing you'll fall into a rhythm and your stutter will decrease over time, it's almost like repetition the more you talk, you will have bad days but the only way to gain comfidence and get a handle on your stutter is by going at it head on. Unfortunately the stutter isn't going away and you won't be doing yourself any favours by avoiding situations where you have to speak.