So, the US number is on my Google Fi account that I pay to keep active and I get a prepaid SIM card for Hong Kong. I can switch the SIM cards if I travel back.
But why? We have two semi-independent networks, one for voice and one for data. The voice one is arcane, region-locked (local numbers cannot easily or cheaply make international calls), and subject to all sorts of restrictions and regulations.
The data network can send and receive data to anyone, anywhere in the the world, for practically free. Why are we even bothering with the legacy voice network at this point? Why can't I get one number that works anywhere in the world? Google Fi kind of does. It's great if you live in the US and travel a lot, but I've heard that if you're living outside the US and use it outside the US too much, they'll shut down your account.
Even porting the number to Google Voice isn't a solution. It only works in a few select countries and of course Google might decide to just shut it down at any time, so who knows.
Internet speeds are more than fast enough now even on mobile in most of the world that you should be able to handle voice calls with a data-only plan. If someone figured out the backend to connect it to the legacy telephone system, to just have one phone number that is independent of a carrier that you can access from anywhere with an Internet connection, they'd make a killing.