Hi,
The snippet assumes you are using jQuery on your website, so if you don’t it won’t work
(you can instead use the last one from Tracking tel: URLs (phone numbers) - #6 by Lukas)
You can always check the Javascript console of your browsers developer tools for errors if you think something isn’t working.
The CSS selector a[href^='tel:'] should target any <a> that has a href that starts with tel:. Again you can verify this by running just document.querySelector("a[href^='tel:']") in the JS console and see which elements it returns.