To be able to receive authentication code via SMS (see more on two-factor authentication), you must first confirm your phone number.
To do this, navigate first to your profile page:
On this page you should have the option to add a phone number to your profile.
In the resulting dialog, enter your phone number. Please include the country code:
In order to confirm that the phone number you enter actually belongs to you, we will send a verification code via SMS. Once you receive this code, enter it to confirm your phone number.
If you do not receive a verification code, please make sure you have entered the correct phone number and try again. If you still do not receive a verification code, feel free to reach out on the support for assistance.