One welcome new feature of Ice Cream Sandwich is the integrated visual voicemail system, but Google isn't providing a built-in app taking advantage of it. Visual voicemail is carrier dependent, so ICS can only provide an API to let developers plug into the system, meaning each network needs to develop an app that works with its own service. Slashgear reached out to Google and confirmed this is indeed the case, and the API should make implementation a little simpler for carries and third parties. So when the Galaxy Nexus launches on December 8th, there'll probably be an app for that. If not, you know who to blame.
Ice Cream Sandwich has visual voicemail API, no built-in app
Ice Cream Sandwich has visual voicemail API, no built-in app/
Ice Cream Sandwich provides and API for the integrated visual voicemail system rather than an app because it's a carrier-dependent service.