"Bring your own phone or tablet" or "LOCKING your own phone or tablet" ... how to solve this ?

Dear sirs,

last week I was visiting US and bought an new phone (btw, payed 100% full price of a new "unlocked" phone to be able to use it anywhere in the world).

after that, I went to a BestBuy store, and bought an AT&T Prepaid "Bring your own phone or tablet" -- great for visitors to the US! to used locally meanwhile traveling thru the country.

for my surprise, when I was back to my country, I've tried to use my local carrier GSM SIM chip on the same phone, but I got a message telling me that the phone is locked !?!? by AT&T to be used only with AT&T in US ?!...

after several hours calling back BB and AT&T, they told me that the phone should be locked for 6 months !! 

how that ? 

I payed for an unlocked phone (full price, in advance) without any kind of discount or future payment ... 1149 USD ... 

I bought the prepaid SIM Card (fully payed also), to work locally without problem ... 45 USD ... 

then by surprise I discover that they "locked the phone the first time I used it with the local prepaid SIM card" ... and nobody, nowhere states that this can happens !!! 

I'm really frustrated both with the information provided by the vendor as well as the treatment of the carrier, because I own the phone, and it was bricked by now ... 

hope to get a light on how to solve this soon.

best regards