Thank you very much for your question on iPhone  unlocking. But you did not write any phone model, for this I am tried to writing  my tutorial such a  way that you can unlock any iPhone if you follow my tutorial.

Now I am telling you the step by step unlocking process of AT&T iPhone mobile.

How to request a for device unlocking of AT&T iPhone?

You can submit a request for your iPhone unlocking if you have eligibility requirement or if you fulfill the requirement of the company. Always remember that Your AT&T phone, tablet, or mobile hotspot must meet the eligibility requirements before it can be unlocked.

Now let’s see the What kind of Eligibility requirements need to fulfill for Wireless, business & non-AT&T customers?  

Requirements for unlocking AT&T phones, tablets, and mobile hotspots.

1. Your device isn’t reported lost or stolen or involved with fraud.
2. You must complete your contract or installment plan (including early termination fees).
3. pay off an installment plan early and then make another unlock request in 24 hours.
4. Your device isn’t active on another AT&T account.
5. If you upgraded early, wait 14 days to unlock your old device. Business customers wait 30 days.
6. Your service must be active for at least 60 days with no past due or unpaid balance.
7. AT&T PREPAIDSM (formerly GoPhone®) devices must have at least 6 months of active service.
8. If you’re military, we’ll ask you to email your TCS or PCS (Temporary / permanent change of station) documents for eligibility. You won’t have to complete installment plans or contracts.
9. If you have a business-owned device, your company must authorize you to unlock it.

The important legal stuff.

AT&T reserves the right to deny any unlock request that it concludes would result in an abuse of this policy or is part of an effort to defraud AT&T or its customers. AT&T further reserves the right to alter this unlocking policy at its discretion without advance notice. If during any one month, more than 100 requests are made from a single source, and if more than 10% of those requests are duplicate IMEI numbers, AT&T will no longer process unlock requests from that source.

If you have eligibility requirement or if you fulfill the requirement of the company, Then you can  Submit unlock request form

To unlock your device to work on another wireless carrier’s network:

1. Visit https://att.com/deviceunlock
2. Select Unlock your device.
3. Read and agree to the eligibility requirements1 to unlock your device.
4. Complete and submit the form.
5. You’ll receive a confirmation email with the unlock request number.
6. Select the link in the email within 24 hours of receipt to confirm the unlock request or the request will be cancelled.
7. AT&T will respond within two business days to let you know the status of the unlock request.

Unlock request approved

If we approve your request, we email to you the instructions to unlock your device and insert the new carrier SIM card.
For iPhone, visit Apple Support: How to unlock your iPhone for use with a different carrier for additional help.

Contact your carrier

1. See if your carrier offers unlocking. Only your carrier can unlock your iPhone.
2. Contact your carrier and request an unlock. Your account might need to meet requirements for unlocking. After you submit the request, it might take a few days to complete. To get your request’s status, contact your carrier.
3. After your carrier confirms that they unlocked your iPhone, follow the steps below.

If you have a SIM card from a carrier other than your current carrier
Use these steps to finish unlocking your iPhone:

1. Remove your SIM card and insert the new one.
2. Set up your iPhone again.

If you don’t have another SIM card to use

Use these steps to finish unlocking your iPhone:

1. Back up your iPhone.
2. After you make a backup, erase your iPhone.
3. Restore your iPhone from the backup you just made.

Get help

You might see this message in iTunes or on your device: “The SIM card inserted in this iPhone does not appear to be supported. Only compatible SIM cards from a supported carrier may be used to activate iPhone. Please insert the SIM card that came with your iPhone or visit a supported carrier store.”
If you see this message, use these steps:

1. Restore your iPhone to factory settings.
2. Contact your carrier to make sure that they applied the unlock in their system.
3. Restore your iPhone from a backup.

To view device instructions for approved unlock requests, you can also:
Visit Check Your Unlock Status.
Enter the IMEI number. To find the device IMEI, dial *#06# or choose Settings > About.
Enter the Request number. This is in the confirmation email you received when you submitted your device unlock request.

You can also download the AT&T Device Unlock Instructions

Unlock request denied
If we deny your request, to become eligible to unlock your device, you can:
Complete your contract or installment plan (including early termination fees).
Pay off an installment plan early and then make another unlock request after 24 hours.
If you upgraded your device before the contract or installment plan ended, wait 14 days after the upgrade to unlock your old device. Business customers must wait 30 days.
AT&T PREPAID℠ (formerly GoPhone®) devices must complete at least 6 months of active service.

Here I have tried to write tutorial only AT&T company iPhone unlocking process. If you are using another company like Vodafone, Three, 02, Virgin Mobile,  Tesco Mobile, and Giffgaff’s iPhone,  then visit this link – All Apple iPhone unlocking solutions for free.

Thank you