Samsung

   This is an AT&T Unlocking guide for Samsung galaxy S8, S8 plus, Note 8 and other Samsung phones locked to The AT&T carrier network.The clear advantage of owning an Unlocked Android Phone is that it can be used with any other Carrier or Network. You can insert SIM card from any service provider on an unlocked Phone and it will start working. Each phone has its own unique unlock code. You will need to contact AT&T in other to get a unique network unlock code specific for your Samsung smartphone.

   However, if your reason for unlocking your Samsung S8 plus/S8 is that you do not intend to use it on the AT&T network, and did not purchase it with that intent, then your best option would be to return the device and get one that will work properly on your network of choice. There is no guarantee that a carrier branded phone, even after being carrier unlocked, will function properly on a different carrier.

Why do you need to Carrier unlock your phone:
Many people need to travel internationally and need to have the phone unlocked to get a foreign sim and number.

How Do i know which Network my phone is locked:

There are 3 different ways to know this:

1.  If your original Samsung box has the AT&T branded logo or if the AT&T SIM card was included in the box. You can also see if your phone has the Network carrier logo at the or when you power it ON/OFF.

2.  You can use an App called Phone INFO Samsung.apk
You can download it from playstore here
https://play.google.com/store/apps/details?id=org.vndnguyen.phoneinfo

3.  The third method is by Checking the simlock status using your IMEI number. To Check Using IMEI Number. go to  https://imei.info website using your phone or computer.
On the next screen, you will see information about the Android Phone belonging to the IMEI Number that you had just entered. Click on SIMLOCK & WARRANTY button.

How to Check your unlock status from AT&T:

Enter your device IMEI and unlock confirmation number.
Enter the Request number. This is in the confirmation email you received when you submitted your device unlock request.
https://www.att.com/deviceunlock/#/status

AT&T unlocking requirements:

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

• Your device isn’t reported lost or stolen, or involved with fraud.
• You must complete your contract or installment plan (including early termination fees). Or, pay off an installment plan early and then make another unlock request in 24 hours.
• Your device isn’t active on another AT&T account.
• If you upgraded early, wait 14 days to unlock your old device. Business customers wait 30 days.
• Your service must be active for at least 60 days with no past due or unpaid balance.
•  AT&T PREPAIDSM (formerly GoPhone®) devices must have at least 6 months of active service.
•  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.
• If you have a business-owned device, your company must authorize you to unlock it.

   I was told that it takes 14 days from the date of purchase for the IMEI number to show up in there system, but after the 14 days the phone will be available for unlock on their website.
https://www.att.com/deviceunlock

   According to customer support, customers have to keep SIM in same phone and account must be in good standing for at least 30 days if its a new account or at least wait 2 – 3 weeks, for their system to recognize it as an eligible account for unlocking. The date on which the phone number is “activated” or “SIM inserted” for network to register the phone + IMEI combination — that is the date from when the 14 or 30 days window starts.

   You do not have to wait 6 months if you paid full retail for a regular phone. Only gophones are locked for 6 months as they are subsidized (not full price).

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

   AT&T allows customers to apply for an unlock via their my AT&T account, or through the AT&T website. With the below info in hand, go to att.com/deviceunlock and fill out a Device Unlock Request Form. Once it’s submitted, you’ll receive a confirmation email to confirm your request, but expect to wait around two business days for the actual unlocking instructions to arrive.

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

If  your request was denied, 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.

How to contact AT&T support:

You can contact ATTCares on twitter directly by using this link below:
https://twitter.com/ATTCares
Or
You can also send a direct message through the AT&T Community Forums Here
Get more information at their official website https://www.att.com/

Common problems and Solution:

Problem:
I went to the At&t website and tried to request an Network Unlock Code.However, it keeps saying” The IMEI you entered does not appear to be an AT&T wireless phone or tablet. Please re-enter the correct 15-digit IMEI number for your AT&T wireless phone or tablet. You can obtain the IMEI number from the wireless phone by dialing *#06#.”

Solution:
If the phone has never been used on the network, the number isn’t in Att’s computer
You will have to send a request by private message here > @ATTMobilityCare and request an unlock. or through the forum below link:
https://forums.att.com/t5/user/viewprofilepage/user-id/3130300

 Source:
https://www.att.com/esupport/article.html#!/wireless/KM1008728