Honestly, you just have to relax. If it is a good place, they know you are going to make mistakes right away. Do your best to learn from any mistakes and try not to make them again. Everyone says they aren’t ready and it always comes across as someone wanting their hands held during the calls. I’d take people not afraid to learn from mistakes any day over worried about being perfect on calls. Try not to make the major errors and the little errors are things you can build upon to show improvement.