When you go to the BIS site (
BlackBerry - BlackBerry | Wireless Handheld Devices, Software & Services from Research In Motion (RIM) scroll to the bottom and pick your provider. For you you have to scroll another page to the bottom on AT&T) there is a "require assistance?" link. Click that and call the number (1-800-331-0500). Have them reset your device on the BIS site and then you should be able to re-register the phone and setup your email on the site. You should be able to change your signature there. It should take about 3 minutes to get your device reset on the BIS site. Stay on the phone with them until you get in.