When you have a user who has a Contact that has been requested to be opted in, you will receive an email from firstname.lastname@example.org providing you with the phone number.
First, you'll want to log into app.textus.com and click on the gear in the upper right corner next to your name and phone number.
Second, you'll click on "Opt Outs" on the lower left side of Settings. This should be the 2nd tab from the bottom.
Last, you'll find the number listed in the Opt Outs screen and click on the trashcan to remove the opt out.