How to unlock an Iphone

I am using ATT for more than 2 years. I got second phone in Jan 2017 on ATT next and then i paid it off in Mar. There was never any condition that was stated to us while buying the phone about any unlocking eligibility.Now when i am trying to unlock my device it says you are not eligible for unlock as you haven’t completed 6 months of paid service as a Prepaid/GoPhone® customer.

When i paid off this device i was still ATT next customer so technically this condition doesn't apply on this device. I am calling ATT customer service from last few days and they assured me that my device will be unlocked. They logged my complain twice still declined. I don't know what do i need to do in this case but if 6 month condition is applicable to my phone as well, then why as a customer we were never informed of that ? It seems like ATT is just focusing on increasing their sales rather than thinking about how the customer is feeling. 

Personally, i am never going to use ATT products one this device is unlocked. I will switch over my carrier

I would like to unlock my old iPhone 6s. I have already upgraded and put my SIM card in my new iPhone. My concern is that on the direction site it says for iPhone you take out the SIM but the phone no longer has one bc I'm using it in my new phone. I guess I'm asking what do I need to do to unlock my old phone?

I tried to unlock my iphone 7 as a non-AT&T user, but got the following error message:

"Because of certain AT&T guidelines, we aren’t able to unlock this device."

How can I get this resolved?

The phone was paid for in full and I am no longer an AT&T customer.

I have been with ATT for more than 10 years.  I am recently in the go phone plan.  I bought the iphone7 with full price from ATT. However the store gave me locked (at that time I did not pay much attention to locked/unlocked phone).  Recently I am trying to unlock the phone since I decide to join my friend's plan with another carrier. However, it has been more than ONE WEEK with many many phone calls to ATT, still, ATT can not unlock the phone! The custom service has helped to send case to the back office (unlocking department) once, which takes three days (2 days for their process and sending me an email, and another day for waiting period).  I did received an email and followed the instruction in the email, however, the phone still could not unlock.  Called ATT again. Custom service told me it was unlocked, however, I can not use the other carrier's sim card.  Called ATT again, custom service don't know why, then helped to send another case to unlocking department.  Still waiting.  

I am wondering why unlocking an iphone takes so much effort?!  I have contacted ATT more than 10 times. The issue is still there.  

What is the other carrier?   Your ATT iPhone 7 is GSM only.  It won't work on Verizon or Sprint. 

The new carrier will be T-mobile.  

Did you set up the phone after installing a new SIM card?  

Or back up reset, restore with ATT SIM card?   Per the email directions...

the itune says "The sim card inserted in this iphone does not appear to be supported"

I phone can active , it lock sim by at&t network why my imei is clean in at&t network . My imei is [edited for privacy-please do not post personal or unique information such as but not limited to full names, employee ID numbers, email addresses, phone numbers, account numbers, etc.] . Could you check for me