How To Format Phone Number With Crossplatform Javascript

<!DOCTYPE html>
<html>
<head>
<title>Phone Validation</title>
<script>
function isPhone(el,evt) {
    evt = (evt) ? evt : window.event;
    var charCode = (evt.which) ? evt.which : evt.keyCode;
    if (charCode > 31 && (charCode < 48 || charCode > 57)) {
        return false;
    }
    if(el.value.length==3||el.value.length==7||el.value.length==13) {el.value=el.value+"-";}
    return true;
}
</script>
</head>
<body>
<p id="para">please Enter your phone number and press the button to test its validity:</p>

<form action="#">
<input type="text" id="phone" onkeypress="return isPhone(this,event)" oncontextmenu="return false"/>
</form>
</body>
</html>