Base on what you describe, you have bought an unlocked iPhone using software. Since you have update/restore it, it is now reset to factory setting as well as updating it's baseband passes 05.13 where you can't unlock it with Ultrasn0w.
There are 2 options you can do to pass this activation screen,
1. Using AT&T sim card to put into your phone, hook it to iTunes and it will bring your phone to home screen. The sim card does not need to be active. You can use old/inactive sim.
2. jailbreake and Hactivate it with Redsn0w. However, please note that it will be a tethered jailbreak because I believe your phone now have 4.3.5 firmware.
Also, even you pass by this screen, you can't unlock it what baseband you have it it now. You will need to install IPad baseband first, then jailbreak it with Ultrasnow.