How can I verify my phone number?

Based on your screenshot, you use mobile phone billing as an Apple ID payment method.

👉 See this Apple article: Availability of mobile phone billing as an Apple ID payment method

Availability of mobile phone billing as an Apple ID payment method - Apple Support (MD)

 -> contact your carrier.

Or change your payment method, see -> Payment methods you can use with your Apple ID

Payment methods you can use with your Apple ID – Apple Support (UK)

You can also use your Apple ID balance as payment method: Add money to your Apple ID balance

Add money to your Apple ID balance – Apple Support (UK)

If your iPhone asks you to confirm your phone number when activating in the U.S. - Apple Support

edit: image referenced in the reply above was expunged, so... this is probably the wrong answer to the question.