unlock AT&T prepaid phone that has been used exclusively on an AT&T MVNO?

Is 6 months of AT&T service prepaid service first a firm requirement?

Yes, that’s a firm requirement. Using on an MVNO does not satisfy the requirement. Even using it on ATT postpaid service doesn’t satisfy the requirement. It has to be ATT prepaid.

You can try the unlock portal, but no doubt you will be refused. If AT&T refuses to unlock it because it's not registered oh, you will have to use it on AT&T prepaid for the required six months.

The message you'll get is something like the phone is not AT&T phone, the IMEI is not recognized as an AT&T phone, or the phone is being used on a different account. 

I tried doing this a couple of days ago using the unlock portal.  I didn't get a refusal, only a "pending".  So I presume the pending will eventually turn into a refusal, or will it just expire?

Thank you for reaching out to us @1f642 ! Allow for us to answer your questions in regards to our Unlock Request Portal!

As previously mentioned in this Forums thread, your phone would have to be considered AT&T Prepaid for 6 months before the Unlock Portal will accept the device. If you need more information on our Device Unlock Requirements, feel free to access the link provided in the beginning paragraph.

Let us know if this information was helpful!

Ty, AT&T Community Specialist

unfortunatley even with 6 months of continuous uninterupted service on att network att will still not unlock a prepaid phone, i have been fighting for several days and it has been one excuse after another which ultimatley ends in them putting a nail in the coffin wanting what they call an account pass code from the phone account which first i find it odd considering prepaid are month to month and not contract and second no one is goign to remember a passcode  from a phone that is 6 years old and the last time it was used was over 2 years ago.  your options would be to skip the useless "customer service" and contact att's office of  president or just pay a third party to unlock it which honestly is far more cost effective than paying 250 for 6 months of prepaid phone service especially if you are only unlocking the phone to sell it.

i did learn one thing though, as soon as i finish my att contract i will be making sure my phone current phone is unlocked even if i have to pay thrid party and telling att to (Edited per community guidelines) off.

I had that happen to Me before,.....Apparently My Safelink service asked for My IMEI and caused it to come up as "being used on another AT&T account" therefore I never could unlock it. I can surely say that if Anyone tries the 99 cent online unlock services, it shall absolutely put the phone in the black list. So Instead, I claimed it "broken" on My protection plan and bought another one. However,....I have in fact had some AT&T phones unlock for Me after the allotted wait time, but it is never a guarantee.