It is illegal for all carriers using gsm-based communication. So that's everyone at the moment and that means that you can't have phones locked when they're purchased. They can lock the phones they're under contracts if you finance the phone. Then they don't have to remove that until the phone is completely paid off. What's been going on over the last 4 or 5 years is the company will give you a subsidy for the phone even if you pay it off in full and then claim that that subsidy is part of a financing deal. So they'll put a fake price and be like the phone cost $1,200 but we'll sell it to you for $800. Then when you pay off the 800 right when you walk out the door you're still getting a subsidy that directly pays for that extra $400 they gave you off that wasn't part of the actual price. If you ever go to change service they automatically use the rest of that monthly subsidy immediately to pay off the phone keep that on the phone since the cost is the same as the subsidy for each month you have the phone untill it's paid off.

If you have Verizon they have been blocking phones even if they're not allowed to do that claiming that any phone not purchased through them or the model number that they sell in the store is not compatible with their Network and needs to be evaluated for security. Then they make it a pain in the ass to get your phone approved to be on their Network and it can take up to 90 days even if it's the same phone just the "unlocked" version with a different model number. This was less of an issue when the FCC rules for GSM based carriers were being enforced, but under Trump and Bush they were not enforcing the rules. And until LTE we had two carriers that were not using GSM based technology so they were not covered by the rules.