I have an AT&T iPhone 4 running iOS version 5.1.1. My contract with AT&T is complete, and they have authorized my unlock. However, the process is to backup and restore from iTunes, according to the instructions they sent.
It seems like an obvious question, but I couldn't find the answer searching. Does going through that process upgrade the operating system? I do not want to upgrade the OS because I am concerned it will make the phone slow.
UPDATE: I followed the instructions from M K, with the results described EXCEPT no iTunes unlock message.
However, even though I had received an immediate response on the AT&T portal with instructions how to unlock, 24 hours later I received an email from AT&T, containing the same instructions as the portal, but now asking me to wait 24 hours after receipt of the email. So I will wait and try again tomorrow.
UPDATE #2: I followed the instructions from M.K. again on the following day, 48 hours after I had submitted my request, with the results described EXCEPT that when I reconnected the phone, iTunes gave me a spinning beach ball, and then crashed. Then I got a message box telling me that iTunes quit unexpectedly. I took the "restart" option. iTunes came up, processed for about 20 seconds, and then gave me the message that my iPhone had been unlocked, in the main iTunes window. No OS update (still version 5.1.1) Thanks!
UPDATE #3: The unlocked phone is, in fact, unlocked -- I took it to Sri Lanka and the Maldives. In each country, I was able to buy a cheap GSM card, only about $3 in Sri Lanka, which let me surf the Internet and make and receive local calls for the week I was there. And I am still running iOS version 5.1.1