AT&T locked my phone that I purchased by myself (not from AT&T)

Your phone was locked according to Apples US Flex activation policy. This activation policy will lock any iPhone other than the Sim free version purchased directly from Apple to the first carrier whose SIM card you insert.

If you did purchase a Sim free phone from Apple, have not had it replaced under insurance or warranty, then the mistake is apples from day one they sold you the wrong phone.   If Apple sold you the wrong phone you should be complaining to them not to AT&T.

If you purchased the iPhone anywhere else, it was locked to AT&T as soon as an AT&T SIM card was inserted under the business plan way before you switch to AT&T prepaid. 

You won’t be compensated for any time lost. Where did you buy the phone ?

Thank you both. I will ask my company where they purchased the phone and whether that is a SIM free phone.

If it is NOT a sim free phone, then how long it can be unlocked? Is it 6 months?

Thank you!

I will ask my company where they purchased the phone and whether that is a SIM free phone.

Don’t bother. If the phone were bought SIM free from Apple, it would already be unlocked. The fact that it is locked means it wasn’t bought SIM free from Apple. Since you have been using on prepaid, the requirement to unlock is 6 months of usage on prepaid. If the phone was paid for in full when purchased and initially used on an ATT postpaid plan, it is possible the phone is eligible now to unlock but since it is being used on prepaid, the unlock portal still imposes the 6 month requirement. You may be able to get it unlocked by filing an FCC complaint.

Thank you. I wonder what is FCC complaint and how can I file one?

Google it.

Thanks. Googled and understood.

I read your previous comment more carefully again - you said that if the phone was paid for in full when purchased and initially used on an ATT postpaid plan, it is possible the phone is eligible now to unlock.

I actually used the new phone with my ATT postpaid plan for 2 weeks before I switched to the prepaid plan. 

In that case, I wonder how long does it take to become eligible?

I have the same issue EXCEPT

6 months

Brother they did me too.... Same here....outright bought this phone not knowing a dang sim card could temp lock an unlocked phone....Dude I was irate.....if the phone is unlocked like you say....take out the sim....put in a sim card that's Not Activated yet....get one from Walmart cheapest you can....but Not Activated yet... Sorry had to make sure it's known......lol..... Your phone will override their options at this point...... Next....I had to factory reset mine quite a few times and quite a few ways thru options thru recovery thru adb and fastboot to get mine where I wanted it......wifi Only and get updates from ur company...... After updates root and custom recovery the phone.... Then go in options and block all ota and fota updates..... Then take out that sim and and put ur prepaid back in and service will come back thru updates will definitely be trying to follow so be aware of that and mine they will look like a factory update but it's prepaid having to come thru and overlay ur phone company uses to let that phone be used by many different carriers......hint unlocked part.....so don't allow it thru...... I even asked a beast of an app manager..... Nothing thinks about moving without my knowing now.....That's how I beat it dude because they did same crap to me then wouldn't give me the code to unlock my own phone......so I dropped em after a year and half.....the whole reason we bought unlocked is to stay unlocked right.....I feel ya main......My Phone My Stuff....if I wanted their crap imposed I'd just paid cheaper and went to them to get the dang phone....all I want is prepay service not ur crap on.....MY PHONE......hit me up if you need help