<script type="text/javascript"> 
function formatPhone(phonenum) {
    var regexObj = /^(?:\+?1[-. ]?)?(?:\(?([0-9]{3})\)?[-. ]?)?([0-9]{3})[-. ]?([0-9]{4})$/;
    if (regexObj.test(phonenum)) {
        var parts = phonenum.match(regexObj);
        var phone = "";
        if (parts[1]) { phone += "+7 (" + parts[1] + ") "; }
        phone += parts[2] + "-" + parts[3];
        return phone;
    } else {
        return phonenum;
    }
}
alert(formatPhone('9131234567')); 
</script>