Also avoid colloquialisms to keep it proffessional and don't forget to breathe, otherwise it means your are talking too fast for the person on the other side to comprehend.
Depending in who the call is to, if you called them and how formal it is try these (as a receptionist most of my day is spent on the phone):
Try and lower your tone a few notes, but sound natural. It makes you more confidant.
Stand up when talking if you can, otherwise keep your back straight and feet firmly on the floor.
If dealing with a complaint, listen, and when suggesting a compromise or solution, smile as you talk. It will relax them and you too!
If you need to, use the 'hold' or 'secrecy' button to compose your thoughts and calm you down.
Have in mind what you want to say before you dial. Run through what you're going to say aloud.
Sound like you're interested in the other person while they're talking even if you're not! And try not to centre a conversation on you.
Think about what you're going to say (but don't read it off a script, that sounds forced) and stand up. Similar to the lying down thing, but easier to do at work!
Before picking it up, while the phone is ringing ... Take 5 seconds and sing loud. Your voice will get on its place.
Especially if you have been in silence for a while or just sleeping.
Lying down is great for casual conversation and the best for flirting, but to make a presentation or pitch over the phone: wear a suit jacket (even if it's over jeans or jammies); style hair, light make up and keep a smile on your face at all times, unless you're firing someone or complaining about cost overruns. You slip into character as you dress and are much more self-confident. Use the aspects of you voice that best match the strategy.
Especially when I'm nervous about a phone call, I will stand on a chair as I talk. Something about it makes me feel brave and disconnected from my nervousness. Also, I imagine the surprise on the face of the person I am speaking with ... priceless! It makes me laugh.