Validate at US phone number input field

validate_us_phone_number.html

 <script src="//ajax.googleapis.com/ajax/libs/jquery/1.11.0/jquery.min.js"></script>
 <script type="text/javascript">
  $(document).ready(function() {
    // FIELD_NAME can be Phone, Cell_Phone or the API name of a custom supporter field.
    $('input[name=FIELD_NAME]').blur(function() {
        if (!RegExp('\\d{3}-\\d{3}-\\d{4}').test($(this).val())) {
            alert('Please provide a phone number like NNN-NNN-NNNN.');
            return false;
        }
        return true;
    })
  })
</script>