phone service to multiple handsets in different rooms.

By "satellite phones," I assume you mean cordless phones, but I'm trying to avoid the radiation from cordless as well as cell phones, and that is why I want to keep a corded landline with my existing corded phones plugged in to extension jacks in several rooms in my house.  I want to know if the Comcast voice service can replace that system and function like my AT&T landline, and if there's a way to get it to function during a power failure the way my current landline does (but cordless phones don't).

How do you connect the extension phone jacks to the Comcast Xfinity system?

Would plugging one end of a phone cord into one of the wall jacks and the other end into the Comcast modem jack do the trick?  Someone from Comcast told me it would last night, but I will have to buy a new modem with a phone jack to find out.

Of course the extension lines are all connected to each other because they can all be connected to the same call at the same time---I guess you're not familiar with the way traditional phone extension lines work.