I have a Canon EOS Digital Rebel XT with the

This means that the communication with the Quantarey lens is not the right one.

You can try in cleaning the lens contacts and the camera contacts with a pencil eraser or a white erase (flip down the camera so no particles get inside the mirror box).

If still you are having problems, then that means the Quantarey lens is defective, and either needs to be service to be repaired, or you will need to get another zoom lens.

Best regards,