Galaxy s5 / Check Imei status outside U.S.- Emergency calls only

Hi friends, I have a problem, I bought an AT&T galaxy S5 on E-bay (I am from Chile), the seller said it was clean and unlocked, and I used the phone for about 8 months and it worked like charm, last month my phone showed a message and I couldn't make calls or get internet (4G), I did hard reset but nothing happened.. restored, updated etc but nothing solved the problem. the phone was without signal for 2 days and it suddenly worked again! until yesterday, I got the same problem... I can see the signal bar, the sim card is recognized (MOVISTAR) but when I call it says "Emergency calls only". I tried with another simcards (WOM, ENTEL, CLARO) and they worked!!! but only with MOVISTAR it's not working... I went to movistar and changed the simcard but the problem continues... now my question is: how can I know if the IMEI is blacklisted or something? and how can I fix it?