You will have to use 2 modems, the new DOCSIS 3.0 for internet and the you have now for digital phone. I think it is good to to have two modems, one because if the internet is down you still have digital phone, two if the digital phone is down you still have the internet, you see its a one two punch lol.
Even with an MTA you're still using part of the same bandwidth, it's just a negligible amount and you'll never really notice it. QOS built into the MTA will throttle your connection enough to allow your calls to be clear and not choppy if you are maxing out your upstream or downstream.