sub some_key_from_tn {
  my $self = shift;

  if (@_) { # called to set value
    $self->{some_key_from_tn} = $_[0];
    # call Telephone::Number method to set value
    $self->{tn}->some_key_from_tn($_[0]);
  }

  $self->{some_key_from_tn};
}