wpf ComboBox can't get focus Move by Tab Key - wpf

Question

It's my GUI.
When I focus on TextBox, I can move focus by Tab key.
But when I focus on Tel No. Text Box, I press Tab Key but focus just stay and not move.
I want to move focus to ComboBox(editable) by pressing Tab key.
.xaml file
<TextBlock Text="{Resx incidentCallName }" Grid.Row="2" Style="{DynamicResource TextBlockSubStyle}"/>
<TextBox x:Name="txtbox_who" Grid.Row="2" Grid.Column="1" Style="{DynamicResource TextInputStyle}" Validation.Error="Validation_Error"
Text="{Binding UpdateSourceTrigger=PropertyChanged, Path=Person, ValidatesOnDataErrors=true, NotifyOnValidationError=true}"/>
<TextBlock Text="{Resx incidentCallNo}" Grid.Row="3" Style="{DynamicResource TextBlockSubStyle}"/>
<TextBox x:Name="txtbox_tel" Grid.Row="3" Grid.Column="1" Style="{DynamicResource TextInputStyle}" LostFocus="txtbox_tel_LostFocus" PreviewKeyDown="txtbox_tel_PreviewKeyDown" Validation.Error="Validation_Error"
Text="{Binding UpdateSourceTrigger=PropertyChanged, Path=Tel, ValidatesOnDataErrors=true, NotifyOnValidationError=true}"/>
<Button x:Name="btn_history" IsTabStop="True" Grid.Row="3" Grid.Column="2" Content="!" Margin="1" IsEnabled="False" Click="btn_history_Click" FontSize="18"/>
<TextBlock Text="{Resx incidentList_type}" Grid.Row="4" Style="{DynamicResource TextBlockSubStyle}"/>
<ComboBox x:Name="cb_type" Grid.Row="4" Grid.Column="1" Style="{DynamicResource ComboBoxStyle}"
ItemsSource="{Binding RelativeSource={RelativeSource FindAncestor, AncestorType={x:Type UserControl}},Path=TypeCombo, UpdateSourceTrigger=PropertyChanged}"
DisplayMemberPath="Code_name" SelectedValuePath="Comm_code_id" IsEditable="True" IsReadOnly="True" Validation.Error="Validation_Error"
SelectedValue="{Binding UpdateSourceTrigger=PropertyChanged, Path=TypeCode, ValidatesOnDataErrors=true, NotifyOnValidationError=true}"
SelectedIndex="{Binding RelativeSource={RelativeSource FindAncestor, AncestorType={x:Type UserControl}},Path=DataContext.CbTypeSelect, UpdateSourceTrigger=PropertyChanged}"/>
App.xaml (for style)
<Style x:Key="ComboBoxStyle" TargetType="ComboBox" BasedOn="{StaticResource {x:Type ComboBox}}">
<Setter Property="FontSize" Value="12"/>
<Setter Property="Margin" Value="5,5,5,5"/>
<Setter Property="VerticalAlignment" Value="Center"/>
<Setter Property="IsTabStop" Value="True"/>
<Setter Property="KeyboardNavigation.TabNavigation" Value="Continue"/>
<Style.Triggers>
<Trigger Property="IsSelectionBoxHighlighted" Value="True">
<Setter Property="IsDropDownOpen" Value="true" />
</Trigger>
<Trigger Property="IsKeyboardFocusWithin" Value="True">
<Setter Property="IsDropDownOpen" Value="true" />
</Trigger>
</Style.Triggers>
</Style>
What is the problem? Please help me.