T.R.
As far as I know (& can find by a quick search on AT&T site), the digital phone is still a "land line", but it runs through their cable.
This is different than a cell phone, where you can call from your phone no matter where you are located - I believe you need to be in your house to use the AT&T Uverse Digital Phone.
The difficulty with calling 911 from a cell phone is that the cell towers triangulate (cross signals to "map" your location) & it is an estimation at best. I had to call from my apartment last month, & the 911 operator couldn't tell if I was in one city or another. I told her the complete address & she was then able to verify.
If you call from a phone connected to your home, whether it is through a cable or the "phone line", it should pull up your exact address in the event that you need to call 911. T.