I came across a pic on reddit of an old device that required people to put a dime into a slot in order to ring your doorbell. If it was a friend, you could refund the dime. If it was a sales person, you could keep it. Of course you could always just knock on the door which defeats the purpose but I was thinking: What would be better is a service that acts as a go-between for phone calls.. people agree to pay you 25 cents to call you. You can choose to refund if you like at some point during the call by pressing a code.. What do you think? I just got my 4th telemarketing call of the day. By the way, I don't own a door bell..
Don't know about phone calls, but for a doorbell just get a cheap CCI camera and an old TV and put the camera just outside your door and rig the switch to your doorbell button, you can even rig it to a VCR and tape the visitors. I have one on my back door, lots of fun plus it makes the undesirables nervous.