Use the custom invalidkey event to flash the input red. To test it, focus the input and type an invalid character e.g. a letter.
<input id="phone" type="tel">
var input = $("#phone");
input.intlTelInput({
utilsScript: "../../lib/libphonenumber/build/utils.js" // just for formatting/placeholders etc
});
input.on("invalidkey", function() {
input.addClass("flash");
setTimeout(function() {
input.removeClass("flash");
}, 100);
});