Google Voice Lite
At least in the US, you don't have to use the voice mail provided by your carrier. I've set mine to use Google Voice Lite (essentially just the voice mail part of Google Voice). This gives me free speech-to-text, which Verizon decided to charge extra for. It also means that it's protected by my Google password (which my phone app has saved, so I'm out of luck if I lose my phone). Good luck hacking it without guessing my password.
Unfortunately, Google Voice is only available in the USA right now, so you need to find some similar service elsewhere. (I hope they'll expand it, but they seem to be instead cutting it back and integrating it with Hangouts, much to the dismay of those who use an adapter to get free home phone service through them.)