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 it has been verified!
Don't forget to claim your free spins for successful verification