Um, actually this already exists. It's called iphone OS X.x. If you have your ringer switch on, when someone calls, it rings, and if it goes to VoiceMail and they leave a message, your phone will play a pre-selected noise to alert you that you have a voicemail. If you have your ringer turned off, vibrate, then once you get a voicemail, it will vibrate once to alert you. I know this, cause my wife calls 5,167 times a day, normally when I'm knee deep in something. The voicemail alert when on vibrate is very similar to a text message alert.