Make Sure your iPhone asking for sim unlock code after insert a non accepted sim card( For Ex. if you are using AT&T sim insert the T-Mobile sim in this)
If yes then follow the steps below:
go to the www.unlockninja.com
and select as per your requiremnet
Model No or Name:-
Country from The Phone Purchased:-
Network on which The Phone Is Locked:-
place order to unlocking confirmation at https://www.unlockninja.com/unlock-apple+phone and wait for the giving time that is 30 min to 2 days
You will confirmed via mail as the phone has been unlocked. Then you only need to follow the steps below:
How to fix sim lock in apple iPhone
How to Unlock the Apple iPhone 6 after getting the confirmation as Unlocked
-Install the last version of iTunes on your computer.
-Make sure your iPhone is updated with the lastest version of iOS.
-Switch ON your iPhone 6 with a SIM Card from a non-compatible network.
-Connect your iPhone 6 to your computer using the Original USB Cable.
-Launch iTunes andwait until iTunes detect your iPhone 6.
-Now disconnect your iPhone 6sand reconnect it after 10 seconds.
-Your iPhone 6 is now Unlocked!
Note:-In the unlikely event that your iPhone 6s is not unlocked, please restore your device following the official guideline from Apple - https://support.apple.com/en-us/HT201252