How to find out if an iPhone you looking to purchase has been marked as lost or stolen by the previous owner. People use imei check service to find if an iPhone has been blacklisted prior to purchasing it from a third-party. if an iPhone is blacklisted potentially unlocking it or using it with the original carrier or even with other service providers will not be possible.
What you need to do first is get your imei this is very easy part, normally by typing *#06# you will get IMEI. Second just go to webpage http://imeipro.info and paste the imei you will check if blacklisted or not.
With this service you can check if an iPhone 3G 3GS 4 4S 5 5S 5C 6 and 6s is reported as lost or stolen (IMEI blacklisted).
Use this service before you buy an iPhone online or order an unlock request.
because if you unlock iphone to icloud you will not be able to make calls.