We initialise the plugin on an input which already contains a full international number. The plugin will automatically select the relevant flag, and re-format the number to national format.
<input id="phone" type="tel" value="+447733312345">
var input = document.querySelector("#phone");
window.intlTelInput(input, {
utilsScript: "../../build/js/utils.js?1541153396801" // just for formatting/placeholders etc
});