Begin with a purpose. The reason I am calling is . Try to respect people's time and aim for a 15-30 second message. Be sure to pause as you say your telephone number. This is especially important when giving your name.
People can hear a smile over the phone, really.
A soft voice won't always be heard, and the listener may miss vital information or nod off. Stand or sit up straight and speak directly into the receiver. Do not cradle the phone in your neck or use a speakerphone. Avoid sarcasm and irritation if you want your call returned. Keep an even temper and state your request.