A little trick for the phone - smile. The physical action of smiling actually has an audible effect on your voice, making you sound nicer, happier, more relaxed. Even if you don't feel like it.

Unfortunately, unless you are good enough to force a Duchenne smile and can reflect it in your eyes, it's easy to get busted for a fake smile in person. So really, this trick works best over the phone. But it does work. Promise.