We have tried many services available on the internet and none of them were able to unlock our locked iPhone for free (we did not try the paid ones).
The problem was solved once we approached the below stated website.
You just select your device type and then you enter the IMEI serial number of it. After that it will take at most 8 hours for the unlocking process. They as well have a desktop version of the same unlocker which is working pretty much the same way.