How to know if your phone is carrier locked
A carrier-locked phone means your phone can only connect to a single network.
This happens sometimes when a phone is on a payment plan, and provider doesn't want the phone being used on other networks. However, they'll sometimes agree to 'unlock' the phone if you explain you'd like to use an eSIM for travel purposes.
Here's how to check if your phone is carrier-locked (tied to one carrier) ...
iPhone: Go to Settings > General > About > Scroll down to “Carrier Lock" or “Network Lock,” which will indicate that there are no SIM restrictions if your phone is unlocked.
Google Pixel: Go to Settings > Network & internet > SIMs, then try adding an eSIM or inserting a SIM from another carrier. If your phone accepts it with no “SIM not supported” message, your phone is unlocked.
Samsung: Go to Settings > Connections > More connection settings > “Network lock status,” which will show whether your phone is unlocked. If you don’t see that option, check Settings > About phone > Status information.
If you’re unable to determine if your phone is carrier locked, contact your provider to clarify your status.