Separate "modem" just for voice + house connects?

Ok, I feel, if this was an option Xfinity would offer themselves, but is there a way to:

A: have a separate, preferably phone service only modem that I can install on a different cable outlet in house?  We have the xFi in the office upstairs, installed before I found out that xfinity couldn't wire the house like our fiber provider did.

B: if answer to A is no, and am required to purchase a second cable modem with voice, will both work in same home?  one would be the barest bones device I can find, I want to think that 2 cable modems, both configured for same service was no good.  but this is early 2k memory knowledge,no idea if would currently work.

C: say there is a solution to A or B, gosh I sure would like to use all that existing infrastructure my home has.  ethernet jacks in rooms, phone jacks etc.  Does anyone have a link or suggestions of a way I could actually route at least voice through the walls in my home to the existing jacks?  I understand this might be outside of 'supported' setups.  but I'm willing to play... More I research, more it seems to make sense to give Comcast back their xfi router/modem thing, and just pick up a gigabit motorola modem and a T-Link wireless router.  Looking at $160 out the door for both.  They will pay for themselves in barely over a year of rentals.  I just need to make sure I can use the voice