Question: how to check if a phone is blacklisted or not?
Answer: Dial *#06# to see the IMEI number on the screen. IMEI is a unique number assigned to your phone. ... 
Enter IMEI to the field above. Do not forget to pass a captcha test. ... 
Verify that the IMEI is CLEAN and the phone is not blacklisted. Now you can be sure whether the ESN is bad or clean.