Keep pushing. The hardest part is over. Stutter through that phone call and make sure you show up to work on time. My advice is not to hold back a single thing you want to say. You won't be able to hide you stutter from a single one of your new coworkers no matter how hard you try, so the sooner you get it out there and start talking the sooner your anxiety about will stop. I remember how daunting this phase of life was, so I know how hard it is going to be to make yourself do, but I'm living proof that you can get through it and your anxiety about it will lessen. Congrats on the job!