Customer notifications
Automatic SMS and email at status changes, and customising the message text.
Moving a ticket between statuses can send the customer a message. Every send is written to that ticket's notification log with the channel, the recipient, the subject and body, the status change that triggered it, whether it succeeded, and the staff member responsible.
If a customer says they were never told, the ticket is where you look.
A message that fails because SMS is not configured, or because the number has opted out, is currently written to the log as a success— the send returns a "not sent" result rather than raising an error, and only raised errors are recorded as failures. So a green row in the SMS log is not by itself proof the message left the building. Email failures are recorded correctly.
Email goes out through your configured mail settings and SMS through your messaging provider. If neither is set up, nothing can be sent. See Email and SMTP and SMS provider.