Opted-out numbers not being added to a list
So you have contacts that texted "STOP" when receiving messages from a list. Now you want to upload them back to that same list. Unfortunately, those contacts will not be able to be uploaded to that list unless they text a keyword associated with that list. This is due to compliance rules protecting mobile users. Keep in mind that you can upload these opted-out contacts to a list that they haven't opted out from.