How to validate if the iPhone is unlocked (after following the above steps, and not having another carrier SIM)?

Connect the phone to iTunes, click on its icon, and click Restore iPhone (NOT Restore Backup). If the phone is unlocked iTunes will display the message "Congratulations. Your iPhone is unlocked."

when will it display the message "congradulations your iphone is unlocked"

Near the end of the Restore of iOS.

Hello Lawrence,

I have recently unlocked an AT&T iPhone 6 using the over the air (iCloud) method, yesterday I borrowed a co workers T-Mobile SIM, I tried it in the iPhone 6 and the iPhone displayed T-Mobile as the carrier on the upper left of the screen along with the signal strength.

Does that verify a successful unlock by AT&T?

Thanks much!

Yes. If you don't get an error message when you insert the SIM it is unlocked.

Got it, thank you so much!

You helped me so much! Thank you! Thank you!!!!

excuse me sir, I did restore and backup my iPhone just like T-Mobile suggested. At the end of the process, iTunes said that there were some changes to my carrier, do you want to continue.. something like that. Does this mean that my iPhone is unblocked?