I wish I knew if this would have helped me on a recent trip out of the U.S. In preparation, I upgraded my older, low-end smartphone to one with a more recent version of Android, NFC (for tap-to-pay), a headphone jack, and support for two physical SIMs. So when I arrived at my destination, I was able to purchase a 30-day SIM for a local phone number and data, but my primary SIM was useless outside of the U.S. so no ac…
Another factor for international travel is whether you phone has the right bands to get signal. My carrier claims to have international roaming, but I look up what bands my model phone has and what the country I'm going to uses and I pretty much would not get signal anyway.
You might not be able to use a provider's extended/rural or dense urban canyon filler cells, but I haven't yet been to a place where I didn't get any roaming connectivity at all.
In some countries (the US included), providers restrict the ability of devices not capable of e.g. VoIP to connect on certain bands (as there is no circuit switched fallback available there, and there's an FCC mandate that calls, in particular 911 calls, have to work wherever data works), but that's usually not applied to inbound roaming guests.