So landlines aren't actually extinct but we are moving in that direction....
I've also noticed a related phenomonon. A LARGE number of people, especially those in the age group of my twenty-something children, use their cell-phones instead of the doorbell!
For example, we had a gal stopping by here to pick up a church key. Robin's phone just rang. My first thought was that the gal was calling to say she had been delayed. Instead, it was her calling to say she was "out front." So Robin met her at the front door.
Also, I can't count the number of times my son has come out of his room to meet one of his friends at our door with the doorbell never ringing (nor any knock).
If I owned stock in the doorbell industry, I'd sell.