Voicemail is provided by your carrier. You should be able to access it from the phone dialer.
If your carrier charges for voicemail an app can't change that. You might be able to arrange it to divert to some other number if you reject or don't answer the call, but if you want voicemail to take a call when you are off network or already in a call then that's a network function, so there's nothing an app on the phone can do to change it.