This is a jQuery plugin used to create formatted phone number values. A list of approved formats can be found in the code comments.

jq.phoneNumberFormatter.js

$.fn.phoneNumberFormatter = function(format) {
  /*supported formats: 
	 * '(xxx)-xxx-xxxx',
	 * 'xxx-xxxx',
	 * '(xxx)xxx-xxxx',
	 * '+x(xxx)xxx-xxxx',
	 * '+x-(xxx)-xxx-xxxx'
	 * */
	var element = this,
		avoidKeyCodes = [8, 39, 37];
	function finalString(originalText, formattedText) {
		if (originalText.length < 4) {
			return originalText;
		} else if (formattedText.indexOf('x') < 0) {
			return formattedText;
		} else {
			return formattedText.slice(0, formattedText.indexOf('x'));
		}
	}
	format = format.toLowerCase();
	element.attr('maxlength', format.length);
	element.on('keyup', function(e) {
		if (avoidKeyCodes.indexOf(e.keyCode) < 0) {
			var returnedString = format.toLowerCase().split(''),
				text = element.val().replace(/[^\d.]/g, '').split(''),
				entry = 0;
			for (var i = 0, j = format.length; i < j; i++) {
				var val = format[i];
				if (val == 'x') {
					returnedString[i] = text[entry];
					entry++
					if (entry == text.length) {
						break;
					}
				}
			}
			element.val(finalString(text.join(''), returnedString.join('')));
		}
	});
}