Be nice and loud and clear! Also, don't speak too fast. Huge part of our job is communication so it's best to make sure they can understand what you're saying. I say this as the master of the mumbles. it's actually a family trait and hard for me to remain clear when speaking. Damn Scottish blood.....
I've really only had one purely phone interview. Other times it was phone interview after a face to face, or phone call to set up an interview...I've had a skype interview before. That was interesting...
As Maribeth said, this is also an opportunity for you to interview them too. Have a list of questions prepared you might want to ask: Responsibilities of the ASM at this company, daily routine, union/non, transportation if its provided or not or is housing within walking distance, any stipends for relocating artists....anything you would ever want to ask someone before relocating to another state.