Customers who opted out by replying short code STOP (ABC STOP) can easily be opted back in.
Manually opting in customers can be done in your Text Integration application by doing the steps below:
Log in to your TI account.
To view the contacts who have opted out, hover over to Reports and select Opt-Out Report.
Select the specific contact you want to Opt in, you can also choose to Select All by ticking the check box at the top.
Once selected, click on the 'Actions' button and select Opt-in.
You can check the list again by refreshing your browser, and see that the selected contact has been removed.