when i've heard my voice (in answering machines, recordings) i've been surprised by some of the sounds. it might make you self-concious, but if you are the kind of person to not let it get to, it could be helpful to listen to your speaking voice somehow. when i did it before it really made an impression, and left me *much* better prepared for professional interactions over the phone (like a job that involved lots of phone calls). i *REALLY* heard the difference of smiling (as above), as well as how close my face was to the receiver (farther away for a clearer, crisper sound with less mouth noise etc - and you'll speak louder, with more confidence), articulating clearly (MORE than in person, since they can't see you) vs. mumbling, intonation, 'crutch words' (like, um, uh, well - they stand out more since they can't see the body language) etc. i think we rely on visuals, and depending on your speaking voice and style you may try to 'make up' for that by focusing more than you normally would on the sound of your voice.
just to bring up another idea that really helped me. also, as above, relax, notes, no distractions, practice.... all very good tips.
you could also be cutting-edge and propose a video call with skype or a similar program =)