How to unlock an iPhone not bought at a carrier?

Whoever you talked to was mistaken or lied. T-Mobile can totally unlock iPhones. They even have a page dedicated to their Unlock policy. https://www.t-mobile.com/responsibility/consumer-info/policies/sim-unlock-policy

Call back and tell them you need to have it unlocked. If they tell you they don't do it, then point them to their own website and ask to talk to a supervisor directly. If there's a reason they won't unlock it, such as the fact it has not been on their service for 40 days, then that's entirely different.

And then tell them, you'll file a complain on them at the Better Business Bureau for lying to you.

If it is locked to T-Mobile, only T-Mobile can unlock it.

Responses to queries at Target's website say:

If you have any further questions, please don't hesitate to contact MyTGTtech at 877-698-4883 every day, between 7am-11pm CST.

If its locked to T-Mobile, then only they can unlock it. Nobody else can.

What probably happened is you bought a device that is under Flex Policy, which means it requires activation with a U.S carrier. At that point it becomes locked to the carrier used to activate it. I believe T-Mobile requires 40 days of use on their network before they will unlock it.

What carrier is it locked to then?

Only the carrier it is locked to, can unlock it.

When you bought it at Target, it should have said what carrier it was locked to.

If it’s locked to T-Mobile and they refuse to unlock it, then it can’t be unlocked. There is no way around that.

Are you actually calling T-Mobile or your local T-Mobile store? Your local store is NOT actually T-Mobile. They can’t help you. You need to contact T-Mobile’s customer service.