One of the lesser known features of the iPhone 2.0 (and 2.0.1) firmware is its ability to play .wav files. If you’re thinking "Big whoop, what do I need .wav files for?", then you’ve never used a voice over IP phone service.
VOIP services like Vonage, AT&T CallVantage, Packet8, MagicJack, and many others all send out your voice mail in .wav format. Formerly, many iPhone users had to go through a tedious process of sending the message to sites which convert the file to .mp3 in order to be able to listen to the audio. Luckily, Apple took note of this issue and included .wav functionality on the iPhone 2.0 firmware.
Check out these screenshots of one of my Vonage .wav voicemails in action: