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.

Maybe they didn't know they needed a phone number to maintain access to the account?

Let's not blame the victim here.

I would never think my phone # was the only proof of identity.

if you immigrate, like I did, but still have some pension funds or saving accounts in your home country. Why would I want a local phone line?