I know how you feel! I work as a community pharmacist and often have to call mostly patients but also GPs and other pharmacists at surgeries. What I would suggest: Maybe discuss your stutter with them in person- a GP I regularly call is familiar now with my stutter, but gps that I don't speak to often can be more difficult as I feel less confident. For me, I used to feel I wasn't giving a professional impression to other people on the phone. Nowadays I've removed this negative thought, I think I was paranoid that people thought I wasn't good enough to be a pharmacist- PLEASE make sure you don't feel like this! Try practicing more, you could always try sending voicemails to someone you know well, or send to your landline phone. Imagine you are leaving a voicemail for the director and see if that helps.