If you have a phone number entered for "SMS phone" and their carrier selected in "SMS Carrier" on the customer's record then you will be able to send an SMS text notification/alert from a repair ticket to update the customer on the repair progress.
To set up an SMS notification, go to Location Settings, then Communications, then Notifications. There, you can "Create Notification Automation" or edit a pre-existing SMS notification.
Once an SMS Notification is setup, you can send it within a ticket. To do this, click the SMS Notification button (the one with the bell icon) on the ticket view screen.