Contacts who choose to opt-out from future communications will populate on the Opt-out tab under Contact Management. 
Contacts in this area will not receive future messages, even if the applicable phone number is in the active campaign’s contact list.
If an administrator clicks the trash can icon on a number, it will remove the number from the opt-out list and make the number eligible to receive messages again.
Keep in mind that if recipients opt-out of messages through the phone provider’s method, then the phone provider will block that number from receiving future messages and the number will not be visible in this area.
If a contact is associated with both an email and phone number, and opts out, both the phone number and email are opted out from receiving further communication.
If an agent attempts to send a 1:1 Text to an opted-out user, a "Failed to Send SMS Message" error will show in the bottom left-hand corner.
The agent can check the Opt-outs tab to verify the contact had opted out.