First of all, make sure you are as engaging as possible. Speak in an energized tone that is loud enough to be heard easily. Use inflection to vary your voice and give meaning to your words. Involve the audience by asking questions and giving them opportunities to participate.
For more help with phone presentations, check out Gilda's audio course, Virtual Presentations - How to Develop and Deliver an Effective Presentation Over the Phone http://www.gildabonanno.com/Pages/VirtualPresentationsRecordings.aspx
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