You can verify your mobile number in two ways: at registration and in your account settings.
Once you've entered your mobile number, click Verify and you'll be sent a four-digit code through SMS. Enter that copy in the box below and then click Enter.
In your account settings
Go to the Profile section of your account settings and find Phone number. If you've already entered your mobile number but not verified it, you should see a yellow notification.
Click the yellow notification, wait for a four-digit verification code sent through SMS, enter the code and press submit.
If you've entered the correct verification code, you should now see a green notification next to your phone number letting you know your it's has been verified!