Of all the ways a phone can fail to accept an eSIM, this is the one people discover latest and resent most, usually the night before a trip. It has nothing to do with age, brand, or how much the phone cost. It's a permission, and only one party can grant it.
What "locked" actually means
A carrier-locked phone is programmed, usually by whoever sold it on a contract or instalment plan, to only accept SIM credentials from that one carrier. It's a restriction written into the phone's software, sitting above the hardware that would otherwise happily accept an eSIM from anyone. Unlocking removes that restriction; it doesn't add any new capability, because the capability - the eUICC chip - was there all along.
How to actually check
Don't guess, and don't assume a phone you bought outright is automatically unlocked - retailers and operators both sell locked handsets, sometimes without saying so plainly. The most reliable check is your carrier's own account settings or support line, which will state the lock status directly. Our compatibility checker also flags this as a question, because it's the single most common reason an otherwise capable phone fails to add a plan.
A secondhand phone deserves the same scepticism twice over. "Unlocked" in a listing is a claim, not a certificate, and the honest way to confirm it is the same account or support-line check you'd run on your own device - before you've paid, ideally, not after you've landed abroad and found out the hard way.