You could use a pay phone to call the operator. You can use it to make collect calls. Hell, if you were industrious enough, you could trick the phone into giving you a dial tone for free. The VoIP ones will probably be harder to trick though

Why? Payphones have never been distinguished by the fact that you had to pay to use them. You also had to pay for the phone in your home.

Payphones were distinguished by the fact that they were located in convenient public places, and if you needed to contact someone, you could use them. That's still true here.

People know what a payphone is, and the service it provides. As he said in the article, he chose the payphone style for what it signals to people - that it is public infrastructure.