How Does iPhone Visual Voicemail Work From An Operator Perspective?

I'm hoping there are some Cell Phone Opera开发者_如何转开发tor gurus here today.

Would anyone be able to explain how Operators achieve the Visual Voicemail feature on the iPhone (and I assume other newer smart phones)?

If a new cell phone operator that distributed SIM cards wanted to utilise the visual voicemail feature on unlocked iPhone's what services need to be in place to be able to support it?

Is there an open spec or is it completely proprietary?

Visual voicemail as implemented by the iPhone and AT&T is proprietary. The 3G data network is used to download new messages. The messages are stored on the iPhone to enable the various features such as scrubbing through the message. AT&T only has to notify the iPhone that new messages are available.