How to make WPF AutoCompleteBox accept more than one Property for filtering

Question

I want to search in multiple fields in AutoCompleteBox, I want to search in ClientName if the user starts typing characters, and search in ClientNumber if the user starts typing numbers.

I searched on the internet and from this Answer I understand that I should use Converter to accomplish what I want, But unfortunately, they do not explain how can I write the converter!

This is the AutoCompleteBox

<toolkit:AutoCompleteBox x:Name="txbClientName" FilterMode="StartsWith" IsTextCompletionEnabled="True"
        ItemsSource="{Binding ocClients}" 
        ValueMemberBinding="{Binding Converter= {StaticResource ClientSearch}}"
        SelectedItem="{Binding ElementName=this,
        Path=ContactPerson,
        Mode=TwoWay,
        UpdateSourceTrigger=LostFocus}" PreviewKeyUp="txbClientName_PreviewKeyUp" LostFocus="txbClientName_LostFocus">

        <toolkit:AutoCompleteBox.ItemTemplate>
                 <DataTemplate>
                      <StackPanel Orientation="Horizontal">
                          <Label Content="{Binding ContactPerson}"/>
                            </StackPanel>
                        </DataTemplate>
</toolkit:AutoCompleteBox.ItemTemplate>

What should the ClientSearch Converter do ??

public class ClientSearchConverter : IValueConverter
{
    public object Convert(object value, System.Type targetType, object parameter, System.Globalization.CultureInfo culture)
    {
        //What should I write here !!!
    }

    public object ConvertBack(object value, System.Type targetType, object parameter, System.Globalization.CultureInfo culture)
    {
        throw new System.NotImplementedException();
    }
}

Any help, please!