However, in reality, not only does the carrier not mind if the phone keeps being used (as long as it’s on the carrier’s network) but the lock doesn’t expire once the customer pays off their plan.
Furthermore the process for unlocking a phone is intentionally made convoluted. Until recently, you couldn’t even figure out which carrier an Apple device was locked to without playing brute-force with all the carrier’s SIMs in the entire world and even Apple support couldn’t be of any help. And when you finally figure out which carrier it is, getting in touch with them is a pain and some have stupid policies like keeping the device on their network for 30 days before being able to request an unlock (a scummy attempt at getting some people to give up and just keep using their network past the deadline, or revenge against someone who doesn’t intend to do so by essentially making their device unusable for 30 days).