I don't have VoIP service through my ISP -- I just used them as an example as why someone might not be able to provide their own DOCSIS device.
For my phone needs, I have Google Voice (this number is given to people I don't know / companies I don't fully trust) and my cell phone (a handful of friends and my family have this number).
Get a magicJack GO for your phone.
$35 a year, and you just plug it in your phone jack and network jack. As a bonus, you can ring your smartphone at the same time.
I love mine.