I just searched for 'Voicemail' on GooglePlay, since I am also interested... There are apps which do what you want, but, their main complaint is bugs and hardware compatibility, so for now, I personally am avoiding them and I wont put any specific links in this thread.
I assume the bugs are related to that the phone manufacturers don't always implement all the 'Doors' needed for a software app to take over and answer a cell phone call from the built in software 'Phone' services in a friendly manner, or, just not enough people want such an app who use it and report on issues so that such bugs are corrected on such a large myriad list of hardware devices and revisions of Android.