"But whether you are roaming still depends on your rate plan in the USA"

Actually, that probably is the same for here. I'd forgotten something. When I got my first cellphone 18 years ago, I was working for KPMG. They bought a huge number of cellphone contracts with 'inclusive overseas' minutes for what was then a very cheap price. As the phones were registered under the individual peoples names, you were allowed to retain and renew the contract when you left KPMG. Ive had the same contract all through and I think it includes 'benefits' that vary from what you would get if you took out a new contract now.