The easiest way to prove ownership is to link the URL and Google Console.

Within AdWords, click on "Linked Accounts" --> Find Search Console in the list --> Enter Site URL --> should show up as verified with a check next to the site

Here's a video to guide you through as well -

After the Search Console connection gets approved, you must go in and re-save the call extension. This will trigger it to go through the motions again, and get approved.

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