A card is selected or maybe a name is thought of. A spectator randomly creates a number. This is NOT a force and they can simply just rattle off numbers from the top of their head. That number is dialed and the person answers and reveals the card.
With this method you can reveal anything and they really do see that the number they created is the number that you are calling. The possibilities are endless and the method is super clean allowing you to be very fair to show that everything is as it should be.
No apps and this can even be done with a borrowed phone. This is Connection by Romanos.