Unlocking an iPhone not currently with any carrier?

I'm travelling abroad in a few weeks, and for a phone I will be using my father's old iPhone (it was his work cell phone, but the company made him switch to a Blackberry for security reasons). The iPhone was with AT&T, and it is 4G. I just ordered my sim card to use abroad, and realized that I don't know if the phone is locked or not. I know to unlock a phone under normal circumstances you call your provider, but it's not currently on a plan or being used as a phone. I know my dad didn't change numbers when he switched to Blackberry. How do I go about doing this?

Just download the free iPhone unlock code generator from the given link below and let it unlock your iPhone without messing with the hardcore techie stuff. I used this program to unlock my mothers's and my iPhone and it only takes a couple of minutes to get the job done.