DOES ANYONE USE THIS TO UNLOCK IPHONES THAT WERE ON PREVIOUS CARRIERS?

The product you linked to is illegal to use in pretty much every country on Earth.

ONLY the carrier the phone is locked to can authorize unlocking it.

https://support.apple.com/en-us/HT201328

Then it's unusual (but not all that uncommon, either), for a carrier to refuse to provide an unlock code for a phone they no longer have a financial deficit with.

Your link was illegal because of what I explained above. If anyone could do that, not just thousands, but millions of people would use it to immediately unlock an expensive "free" phone and go with a cheaper carrier without ever bothering to pay what they owe the carrier who sent them the phone another dime. Sure, the carrier could go after the person for the fees they owe, but when you're talking about hundreds of thousands of claims or more, litigation and recovery could cost more than what's owed.

Short version, you'd be illegally breaking a binding contract. That comes down to, "See you in court.", have you arrested for property theft (the phone isn't really yours until it's paid off), having your credit rating completely ruined by having your name and contact information sent to a collection agency, maybe have your wages garnished to recover the value of the phone, and more.

In your case, the phones are paid for. Most of the above is in regard to the much larger percentage of phones that aren't.

Edit: A bit more explaining some of the reasons a carrier may refuse to provide you with an unlocking cold.

• If you are in contract with your carrier, they will not unlock your device until the contract has been finished.
• If you have gotten a phone on an installment plan your carrier will not unlock your phone until the device is paid off and you officially own the device.
• If you have leased a device through your provider, they will not unlock the device as you do not own the device.
• The most common way to have your device unlocked is to pay for the device once your lease is up or buy out your lease and the phone.
• If you have bought a prepaid phone outright, they will not give you the unlock code. The carrier will not unlock the phone for one year after the purchase and activation of the prepaid phone.
• If you have a prepaid phone from Boost or Virgin Mobile, the carrier may require the phone to be active with their service at least 6 months before they will agree to unlock the phone. This is generally due to the phone being sold well below retail value.

You didn't mention how you paid for your phones, but a prepaid phone may be why. These types of phones are sold below their value. They make their money back on it by having you continue to add minutes to the phone's plan. That's why they won't release a pre-paid phone for at least a year.

Such devices make multiple illegal calls to emergency services numbers in an attempt to bypass the carrier lock.

If they're refusing to unlock the phone, that's normally because you haven't paid off the phone yet. There is, no matter what the TV ads say, no such thing as a free phone.

You actually pay for the phone over the length of the contract of two or three years. Why do you otherwise think your basic monthly fees are $80 or more a month?

They'll let you terminate the contract early. But of course there's still the balance on the phone to pay off. That's mostly what the early termination fee covers.