Contacts who choose to opt-out from future communications will populate under the Contact Management section, on the Opt-outs page. 
Contacts on this page will not receive future messages, even if the applicable phone number is in the active campaign’s contact list.
If an administrator selects the trash icon next to a phone number, it will remove the phone number from the opt-out list and make the number eligible to receive messages again.
Note: 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 a 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 appear in the bottom left-hand corner.
The agent can check the Opt-outs page to verify the contact had opted out.