My iPhone can't be opened.

Not sure what you mean by “untie” but if you can’t get past the screen lock security code, contact Apple, not ATT, or try what is detailed here: https://support.apple.com/en-us/HT204306

Hello Taiom, we're here to help get your iPhone opened.

As the ACE @sandblaster mentioned, if you can’t get past the screen lock security code, you would want to contact Apple. 

If you're trying to get the phone unlocked, all unlock eligibility requirements would have to be met before submitting your request.  

Let us know if this helps. Feel free to reach back out to us if you have any other questions.

Jeff, AT&T Community Specialist 

Do you mean screen locked as you can't get passed the Home Screen login or locked to AT&T (and you can't unlock it)? Please be specific. Screen locked would be a call to Apple. AT&T Unlock would be a call to AT&T.

@Taiom  This is a public forum, not AT&T Support, so please use the three dots to the right of your post and EDIT out your IMEI number. Scammers/spammers/phishers look for info like that.

Where did you purchase your iPhone? From Apple, AT&T, or a third party.

I bought it from a third party, but when I bought it, he didn't tell me that I had any arrears. I don't know anything about the iPhone and it's my first time to use it. I'm sad now because I don't know how to find customer service to apply for unlocking. If anyone can help me, thank you very much

@Taiom  That is the problem with buying a phone from a third party. If there is an unpaid balance on the phone then only the ORIGINAL owner can pay it off. Otherwise, AT&T will eventually blacklist the phone and it won't be able to be used on any network. AT&T won't tell you how much is owed nor will they allow you to pay it off. Sorry to say it but it appears that you've been scammed. With an unpaid balance there is nothing that you or AT&T can do.

But my mobile phone here is not a blacklist. It's just a whitelist of arrears. Can I solve this problem by paying?

@Taiom  You're not understanding. The previous owner owed AT&T money for the phone. They chose not to pay off what they owed and instead sold the phone to you. AT&T is still owed money from the original owner. You can not pay off to AT&T what is owed to them. Only the original owner can. This is a common scam where the owner not only scams AT&T for money that is owed to them they scam the person who purchases the phone from them because it can't be unlocked until paid off. It may take AT&T some time before they blacklist the phone so until then, they won't allow the phone to be unlocked for use by you.