@jameswallis Well, my wife phones me from the other end of the house, i.e. use mobiles as an intercom.
For the front door bell, I made sure to get a wired version, running on PoE, so no battery/wifi issues (but still vulnerable to complexity due to networking).
@sgryphon Our doorbell communicates with the ringer by radio waves, but it generates its own power--a single press creates enough juice to send the signal. The ringer doesn't work in a powercut, of course.