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.
Just an FYI for anyone else coming across this. WordPress runs jQuery in compatibility mode, so you will typically need to specify jQuery instead of $ as shown in the following example:
jQuery("a[href^='tel:']").on("click", function (e) {
_paq.push(['trackEvent', 'contactform', 'called', this.href]);
});