How to unlock phone I purchased used.

Anyone can request an unlock using the online unlocking portal.  Doesn’t matter if it’s used, or whether you’re the original owner or not.  All that matters is that the phone meets the eligibility requirements.  However, if it’s not eligible, and it’s a second hand phone, it’s likely that you won’t be able to make it eligible.

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

The thing is I do not know the history of the phone. I do not know if it is active on another account or if it’s paid off. All my phone tells me is that it is sim locked and when I click on that this is what it says.

Hello @Madisonelle, we're happy to help with information to unlock of your device. 

If a phone is purchased from a non-AT&T authorized dealer, third party sellers or sites like eBay, AT&T may be unable to unlock the device. Before purchasing the device, make sure the seller performs the unlock request. In some instances, you will have to contact the seller to perform the request. If the device was purchased by another provider, they should have online resources to unlock the device.

Here are the General Requirements for All Unlock Requests:

You don’t need to know the history.  If it’s eligible to be unlocked, it will be unlocked.  If not, you’ll usually get an error message telling you the reason it was denied.  You have nothing to lose submitting a request.  It doesn’t cost anything.

Trying to unlock an AT&T Lumia 950. However when I enter the IMEI number it tells me that the phone is on an installment plan that isn't fully paid for. I bought the phone used on eBay so I do not have a payment plan. Can I get this phone unlocked?

@wlroth  The fact that YOU don’t have an installment plan isn’t relevant.  The original owner bought it from AT&T and didn’t pay it off.  Until the original owner pays it off, it won’t be unlocked, and there isn’t anything you can do to pay it off.  Try to get your money back.

Thanks for the quick response. That's what I was afraid of.