For decades there has been endless policy wrangling over whether “unlocking your phone” (removing restrictions allowing you to take the device with you to another carrier) should be allowed. Giant carriers have generally supported onerous phone locks because it hampers competition by making it harder to switch providers. Consumer rights groups and the public broadly support unlocked devices.
When shopping for phones to buy outright, I remember seeing that you can buy phones locked to specific carriers. Supposedly different carriers use some different bands and so the carrier specific version has the corresponding antenna/receiver.