Even with today's 1.1.1 iPhone firmware update, it still can't play the .wav files I get via email from my company's voicemail system. I now see a nice new Quicktime logo next to the attachment, I can click on it, and Quicktime launches... but it then immediately errors out with "This movie could not be played".
After digging a bit deeper, I discovered the problem is the iPhone can't play .wav files that use the particular codec our voicemail system is using: G.711 (which apparently is a very common codec to use for telecommunications). "WAV" is just a packaging format (like AVI), it can have uncompressed or compressed (even mp3!) audio inside of it.
You can hit Wave File Compression Codecs Compared to see a variety of common .wav file types, and try each to see which will play in your iPhone. Here's what I found:
|Codec||Works on iPhone|
|11,025Hz 16bit PCM||yes|
|8000Hz 16bit PCM||yes|
|11,025Hz 8bit PCM||yes|
|8,000Hz 8bit PCM||yes|
|8,000Hz u-Law (aka G.711)||no|
|11,025Hz 4bit ADPCM||no|
|8000Hz 4bit ADPCM||no|
|8000Hz Mp3 16k||no|
|8000Hz Lernout & Hauspie SBC 12k||no|
|8,000Hz DSP Group TrueSpeech||no|
|8000Hz Mp3 8k||no|
|8,000Hz Lernout & Hauspie CELP||no|
So it appears that the iPhone can only play PCM .wav files, which are probably the most common you'll find on the web. However Vonage customers are still out of luck, David confirms Vonage voicemails also use G.711.
Oh well... it's a step up from not being able to play any .wav files, maybe next update will support a broader range of codecs.
Update January 16th, 2008: still not working as of the 1.1.3 update.