Have all the documents you need to hand before you dial number.
Speak clearly so that the other person can understand you easily
Always confirm each point of information you are given. Don't pretend that you have understood when you haven't.
Speak in a polite and friendly voice the client can't see what a nice person you are.
Behave in an efficient way. Even if you are just taking a message.
Make notes during a call - don't rely on your memory. Read all the important details back to the client from your notes.
Let the other person finish what they want to say- don't interrupt them.
Sent a follow- up fax or letter to confirm any important details (especially prices and dates), so that you both have a written record of them.