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 - https://www.youtube.com/watch?v=-z0m2nHcv44&feature=youtu.be
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.