Your voice, your words, your attitude, and your body language all contribute to the image you project. In addition to visual cues, auditory cues—like the tone and quality of the voice, and choice of words—contribute to the all-important first impression.
On the telephone, however, the benefit of the visual cues is lost. The person on the other end of the telephone forms an image of you based solely on the tone and quality of your voice, and your choice of words.