The public telephone network is incomparably more interoperable than something like WhatsApp. You cannot self-host it, but you can keep the exact same phone num

The public telephone network is incomparably more interoperable than something like WhatsApp. You cannot self-host it, but you can keep the exact same phone number while switching providers (which is even more flexible than email), phone manufacturers and OSes, and "client" (phone modem) implementations at will. No such thing is possible with WhatsApp. Various implementations come from the same company, do not have feature parity (e. g. phone calls are not supported on certain mobile OSes), and definitely don't allow anything in the way of integration with the outside world. A PSTN connection can be hooked up to a PBX to let you automate voice and all kinds of other things, while SMS can be synced to the computer using an app with the appropriate permissions. In short, PSTN is malleable, while WhatsApp is not.