Unlock failing

I had my IPhone 3GS unlocked by contacting AT&T through website long time ago. Finally, I started to try it, yesterday. However, after finishing restore it is stuck on No Service even if there is no SIM card in it. iTunes says "There is no SIM card installed in the iPhone you are attempting to activate. Please disconnect and insert a SIM card in the iPhone". I don't have an original SIM and neither can I get another AT&T SIM but it is not saying me to insert correct SIM. There was working SIM from another operator when it was restoring. I tried to flash it in DFU mode without SIM and it was stuck in Searching... mode when it was powered on. Inserting SIM doesn't change it and iTunes again says me to insert one.