Regular Expression for Fax number validation in ASP.NET

<input id = "txtFax" type="text" />
<input type="button" onclick="ValidateFax()" value="Validate" />
<script type = "text/javascript">
    function ValidateFax() {
        var regex = new RegExp("\\+[0-9]{1,3}-[0-9]{3}\\-[0-9]{7}");
        if (regex.test(document.getElementById("txtFax").value)) {
            alert("Valid");
        } else {
            alert("invalid");
        }
    }
</script>