To enable phone 2FA, go to your account settings and click the Set Up button next to the "SMS 2FA" field.
In the following screen, type in your phone number and click "Send Text Message". You'll then receive a text message on your phone. This message contains a 7-letter unique code. You'll need to type this code into the Phone Code field at the bottom of the screen. You'll also need to provide your password.
Once you are done, make sure to enable Phone 2FA in the account settings. You can do so by clicking the blue button next to the "SMS 2FA" field.