There are two ways to validate your phone number.
1. Via the Signl4 mobile APP
Login to the Signl4 mobile App with your account. Then click on SETTINGS (little gear on the top right) and go to Profile. There you can put in your phone number and verify it.
2. Via the Signl4 Web GUI
Login to the Signl4 Web GUI with your account https://account.signl4.com/manage. Then click on your name on the top right and go to My Profile. Under ALERTING you can verify your phone number.