This is more so a phone provider specific system....
Each Phone Provider is given the source code to android, and they make changes as they need to.
Such as, using 1's 3G services, where to send signals for SMS, etc, etc, etc...
This would fall under all that, and therefore its fairly impossible to get such information currently, unless there is some form of an API that the voicemails follow off of, but even reading up on the Android API, I do not see anything either.