iphone i bought used as a replacement, i went to update it and will not activate... help

Bought a used 3GS iphone, tried to update os prior to insalling backup frrom my (other) damaged phone, erased everything and ow says cannot activate, its at the set up screen like first time setup but cannot get past activate, trying for hours...any help? PHONE MAY HAVE BEEN UNLOCKED?> I DONT WANT THAT, HOW CAN i RESTORE TO NEW SETTINGS?