Your phone number is easily verified using the Air Control mobile apps.

From the main airspace map, tap the menu in the top left. Then, top on your name above the black box that shows the account. Tapping the Verify button from your profile will set the verification code. You should receive the code to the same device, and copy the code into the app.
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If you have any troubles, you can also do the verification process from the web:

https://air.aloft.ai/
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Head to Manage > Users > click your name > and look for the verify button next to your phone number on the open right-hand panel.