Outsourcing "unique identifier that costs some money to game yet can be immediately used for interaction" to cellular network providers is known to be a reasona

Outsourcing "unique identifier that costs some money to game yet can be immediately used for interaction" to cellular network providers is known to be a reasonably effective and generally globally scalable solution. WhatsApp relies on it. Apple accounts and I suppose therefore iMessage relies on it. I hear Signal relies on it too. That alters the graphic somewhat...