Project Fi is already network agnostic with one phone number but multiple networks
Google's Project Fi is already doing this (with eSIM on the Pixel 2 and standard SIMs in other phones) and despite its faults and flaws works incredibly well. Largely seamless switching between T-Mobile, Sprint, US Cellular and will roam on Verizon when nothing else is available. The SIM card has IMSI profiles for each network. In some places it will also roam on AT&T. It switches automatically between networks and leverages whatever is best in an area. Then, when abroad, its one price, $10 per gig and .20 per minute for phone calls, texts and pic messages are unlimited. Also, if there's a Three network in the country being visited and/or roaming on Three would provide a better experience, the SIM has a Three IMSI and will utilize that network all without any notification or involvement by the user. If one wishes, they can force the phone to any of the networks, although if in the US, one cannot force the Three SIM profile.
Biting the hand that feeds IT
