how you'd design a state-of-the-art smartphone
Use the camera.
Have a massively complex multicoloured QR code on a bit of paper that you take a picture of. The phone then connects to the service and says "let me be that number.
Point two phones at it, they both become that number like an old-fashioned plan 1A domestic extension. Both ring together - even in separate countries - and the first one that picks up is online.
Point the same phone at several certificates, and it will accept calls for all those numbers, and ring out on any one.
Buy a prepay at the garage, get a top-up you point the camera at. Pay for top-up at a cash machine, and photograph the rapiidly changing screen,