telephone interview etiquette

Don't smoke, chew gum, eat, or drink. Do keep a glass of water handy, in case you need to wet your mouth. Smile. Smiling will project a positive image to the listener and will change the tone of your voice. Speak slowly and enunciate clearly.

Also Polish Up Your Phone Communication Skills and Send a Thank You Note

Try to be relax your self and speak in a same pitch where interviewer can understand what you respond...