Even before GSM there was NMT, that demonstrated the possibility of a single standard for use in multiple nations. NMT was analog though and thus much more con

Even before GSM there was NMT, that demonstrated the possibility of a single standard for use in multiple nations. NMT was analog though and thus much more constrained in capacity. And GSM brought us the SIM card, that made switching phone or service provider as simple as moving or replacing a card.