Firstly, go to your Profile -> Edit Details to add your phone number along with your mobile provider name:
To confirm your phone to receive text messages please click Dashboard from the top right side of the app, and click the link which says Send a confirmation text now.
Once you receive the confirmation text, please submit the confirmation text into the box given on the dashboard and press the Submit button (as appearing in the screenshot below).
Please feel free to let us know of any feedback by emailing firstname.lastname@example.org. Happy Scheduling!