I just signed up for voicemail from eVoice 'cause I need a number for a few weeks, and I was dismayed to discover that not only do they have a proprietary audio format, but they don't have a Mac player -- instead you have to write in and ask 'em to change something.

So instead, I installed sox and fiddled around for a while, and discovered that the files are GSM format (whatever that is.) So to convert 'em from .evc to .wav, the command is simply sox -t gsm foo.evc foo.wav.

Getting sox installed is up to you; I used pkgsrc to get it working on MacOS X, but I'm weird that way.