Go to your profile page.
Press the “Edit” button and fill the "Phone number" field. The number should be filled in international format (e.g. +441234567890).
Press continue, and wait for SMS with the code to come.
Enter the code and press the "Confirm" button.
If SMS didn't come:
Make sure that you don't have any SMS spam blocking software or mobile provider services.
Try to use the "Resend SMS" function up to 3 times.
If you didn't receive an SMS three times, the button "Call me" will be available for you. Press it and be ready to receive a call from a bot that will tell you the code. The call will be in your language.