You'll have to have the contact text in from their phone to be added back. We don't allow you to opt someone back in. It has to be done by the owner of the phone and number.

Long Number (Local or Toll-Free) Opt-in
On a long code, have your contact text the word START to your number to be added back.

Shortcode Opt-in
On a shortcode, you'll need to have them text a keyword in your account. If you don't have a keyword, you can add a free one under TEXT MESSAGING > AUTORESPONDERS.