When ever I am on a one on one or over the phone, I am always professional and well spoken. Speak your strengths and show your ability.
One plus also is that I have a catalog or photo album (if you like) to show clients jobs I have done. It shows before and after shots and completed jobs. Pictures are worth a thousands words!!
It allows you to speak less if you are uncomfortable with that part of the job.
Me, I talk too much.