[Q] What voiemail app to "unlocked" phones come with?

I know this is a trivial question, but I've never bought an unlocked phone and was just thinking....What voicemail app to these phones come with?

Since I'll be using it on At&t, will I be able to download the At&t VVM app....... Or will I need to use the, dying, Google Voice? Or will it have a built in Voicemail app?

Like I said, very minor question, but one I wonder about?