Mobotix has a pretty slick "doorphone" piece that we are considering for a project that will "call" an app on a smartphone. Single CAT wiring, or existing 2 conductor can be utilized. I have seen the unit briefly, but have not had a chance to hook it up and use it. Sounds like it might do what you are asking. Worth a look. T.hey make some solid stuff, those Germans.
"When I eat, it is the food that is scared." - Ron Swanson