HTC Imagio Unlocked but can I use it?

If it's unlocked, you should be able to use it. Is it possible that your friend's AT&T SIM card was from a pre paid and they didn't have any credits to call with? What you need to do is put your SIM card in the back of the phone and it will be ready to use. If it called with your friends SIM you have a working, unlocked phone regardless of wether or not they had credits.