Sounds like your iPhone was previously hacked/modified/jailbroken
to work with T-mobile and the update has relocked the iPhone to the
original wireless carrier. You must contact the original carrier
to see if they offer unlocking and if you qualify.
What does it say when you look at Settings=>General=>Carrier?
Be advised that the previous modifications to change from original carrier
to your T-mobile may have rendered the software unable to be
officially unlocked or even restoreable to pristine condition.
Google may provide some direction, but there is not help for modified iPhones
here per Terms of Service.