How is iphone unlocked?

Question

iPhone

I came across a phone that was locked to some operator. And it worked when I bought it, I did not reset it. Now I did a reset and got a SIM card lock. I haven't come across loki before. tell me how to fight Can I flash on some kind of unlock version of the firmware, jailbreak or what else is there? If so, I would be grateful for a link to the info. It's embarrassing that repair services offer this service, so there is an opportunity .. ready for a jailbreak, but I understand that from version 11.2.5 you can't install it?
Iphone 6s (A1688 according to the case)

Answer 1

Thank you all for the zeal and so many replies, but I've already researched the question myself.
Unable to unlock verification takes place on the authorization server by imei, which means you need to change imei, which is already wired in 4 boards. It’s also not a fact that you won’t get on someone’s active imei and they won’t start to conflict (a fictional one, of course, won’t work). It is possible to allegedly put a jailbreak and put some kind of software for substitution. But it's not clear what to do with a non-activated phone (after a reset, for example). for jailbreak, you already need a working phone + jailbreak is not available on current versions of ios, and they do not sign the old ones. those. if you are on the latest version, then you cannot roll it back. Apple doesn't want this. Of the options, unlocking through online services looks more or less realistic. I believe that repair services do exactly this for already broken online services. at the moment i am waiting for gsx report from imei-server. ru which should tell which operator my phone is attached to, there are also prices for "unlinking". The question is the price. it varies depending on the operator from $10 to $200. Of course, it makes no sense to give another $ 200 for the dubious service of some online service. but $ 10-20 in my opinion is quite possible to risk. you can still contact the operator and ask to untie if the contract has expired (1-2 years).
PS
Saved a pancake on the iPhone. I think this is where my three-year history of communication with them will end. It's a pity for a good device 60+k, and attempts to save money lead to such failures. What for you need to use the services of the company against its will ..