I have been having a problem receiving my voicemail on my iPhone. My wife would leave message on my cell phone, but my phone would show no indication of a waiting voice mail message. Also when I call in to get my voicemail, nothing appears their either. I think that this is a problem with AT&T and not my iPhone.
Google made it available to use Google Voice with existing cell phones. With this you can now have your voicemail sent to Google voice, and you can listen to it from your phone or your computer. Also Google Voice will create a transcript of your message, and you can received that via email or sms.
I have been using Google Voice for a little bit, but this functionality allows me to receive messages from callers that don't know my Google Voice number in my Google Voice inbox.