Iphone 6S unable to activate

I am getting the below message...

"Your iPhone could not be activated because the activation server is temporarily unavailable. Try connecting your iPhone to iTunes to activate it, or try again in a couple of minutes.

If this problem persists, contact Apple Support at apple.com/support."

My brother traded my old iphone 6S and got another 6S for crazy reasons. He used it for couple of months and after the Apple update he couldn't able to use the phone and the above message is occurring. If this phone is locked we don't know the original carrier with whom it was locked with. Any help is greatly appreciated.