1. Control. If you feel your speaking is outwith your control then you will struggle. Find ways to be in better control over your speaking i.e. breathing exercises and speaking slowly. I can hazard a guess that your work is fast-paced hence the perception is to speak fast. 2. Other people. I mean when the focus is on somebody else, or the reason you do something, is bigger than yourself. Almost as if you're calling/representing on behalf of somebody else. Sometimes adopting the mentality of calling on behalf of somebody else can force you to step it up. Because your phone call is for something bigger than yourself. To some this would add pressure, and to others this would take the pressure off you. It'd also desensitize you when it comes to doing this again. 3. Mindset. Always have the focus on staying relaxed and composed. You can still be relaxed and speak fast. That's what I've gathered from reading your post. Good luck!