Here is a third party app note showing which .wav implementations are playable on the iPhone
< <a class="jive-link-external-small" href="http://">http://www.kehlet.cx/articles/156.html>
Unfortunately, the G.711 format is widely used for telephony and voicemail and currently does not play on the iPhone. I encountered this when my AT&T home voicemail would not play when forwarded to my AT&T exclusive iPhone. (so much for corporate compatibility)
One workaround is to forward messages to www.iphoneconvert.com. This service changes the file format and automatically forwards to the iPhone.
Hopefully, Apple will address the needed G.711 audio support in software 2.0 (July 2008) or in a future update. This would eliminate the need for the extra forwarding workaround.