[snappieT]

It can be costly.

I moved from Ireland to the US and kept my Irish number active - the cost was a €5 topup every 6 months.

Going in reverse is much harder - a lot of the budget phone providers in the US don't have any roaming offering. Best I can tell, you really need to have an account with a real provider, and that realistically looks like $20/mo (Google Fi), 20x more expensive than the reverse.

[ThePowerOfFuet]

Then it sounds like changing bank is a better answer for many.

[snappieT]

This oversimplifies the situation - if every US bank uses SMS and you want to retain a US bank, what do you do?

[ThePowerOfFuet]

That's such a huge "if" that an alternative immediately came to mind:

TransferWise doesn't require a US phone number, but you can have a US account number with them.