Need old LG K20 phone unlocked

You're supposed to try the portal 

https://www.att.com/deviceunlock/

Given the age of the phone and you indicating you're not a customer anymore it's likely the IMEI is no longer registered. The likely response is, denied because it's not an AT&T device, or IMEI not recognized. 

You might wind up having to call AT&T.  If that phone has not been updated in several years, it may not be possible to update it now. It would work on AT&T or any of its MVNOs. It is not going to work on most other service providers 

Understood - it's worth a try 😊

Thanks so much for answering so quickly.

Hey there @izzyjava,

Thank you for reaching out to us. We are here to help you in any way possible.

Learn what you need to do to unlock your device and the requirements here: https://forums.att.com/conversations/other-phones-devices/att-device-unlock-help-solutions-faqs/5defc295bad5f2f606ddc2ce#:~:text=%20How%20To%20Request%20A%20Device%20Unlock%20,non-AT%26T%20Customer.%205%20Follow%20the%20prompts.%20See%20More.

But if you are not the original owner of the device and are denied a device unlock, you will need to reach out to the original seller/ owner. AT&T is unable to assist.

Please let us know if you need further assistance on this.

Thank you for visiting the AT&T Community Forums.

David, AT&T Community Specialist.