1. Create an outline of all of the key points you want to bring up to the person you'll be speaking with on the phone. The outline can be as detailed or as sparse as you feel you need, but having something to use as a checklist of sorts, along with any additional details that are pertinent to the conversation, will help give you a boost when it comes to stumbling over your words. And really go over your outline, because familiarity with what you're speaking about will help you instill confidence within yourself, which will help you sound "smooth."
2. Work on leading the conversation without dominating it. There's a fine line there, but with time and practice you'll get get the hang of it. Your main goal here is to keep the conversation focused by sticking to your outline. This will help remove any awkward pauses.
3. Smile when you're speaking. Most people sound friendlier when they smile when they talk, even if you're not sitting in front of the other person.
Hope this helps!