Although some facilities send the check digit, it is not a requirement. You can calculate the check digit on your own. Here is some guidance.
A check digit is used to detect errors in an intermodal shipping container number. The seventh digit of the container number also known as the check digit, is calculated off of the four letters and six digits of the container number. As long as the container number follows the ISO standard, the container number can be entered into a check digit calculator to determine if it is a valid number.
For more information on ISO 6346 intermodal (shipping) containers and check digit calculations, click here.