When you sound professional over the phone, you're seen as a professional even if the prospect hasn't met you yet, and even though they can't see you your image will come across loud & clear.
The truth is there are a number of things you can do to maximize how persuasive you are over the phone, some are simple tricks you can start doing right now...
I've compiled the most effective ways into my white paper, "7 Ways to Be More Persuasive On the Phone".