If you have a client that does not receive the SMS code, there is a way to manually generate a one-time code. Select the folder that has been shared and go to the Sharing menu. Click on the User options menu and select the user that has not received the SMS code. Click on the "Get Login Code," and you will see the code in the top right corner. You will then need to mail or call the client to pass on the code. It is vailed for 1 hour.
Of course, the best is to move the client over to use an authentication app, more about this in the link here.