<DataTemplate x:Key="PhoneNumberTemplate">
  <StackPanel Orientation="Horizontal">
    <TextBox Name="tbPN1" />
    <TextBox Name="tbPN2" />
    <TextBox Name="tbPN3" />
    <TextBox Name="tbPN4" />
    <TextBox IsReadOnly="True">
      <TextBox.Text>
        <MultiBinding StringFormat="{}+{0} ({1}) {2} - {3}">
          <Binding Path="Text" ElementName="tbPN1" UpdateSourceTrigger="PropertyChanged" />
          <Binding Path="Text" ElementName="tbPN2" UpdateSourceTrigger="PropertyChanged" />
          <Binding Path="Text" ElementName="tbPN3" UpdateSourceTrigger="PropertyChanged" />
          <Binding Path="Text" ElementName="tbPN4" UpdateSourceTrigger="PropertyChanged" />
        </MultiBinding>
      </TextBox.Text>
    </TextBox>
  </StackPanel>
</DataTemplate>

public class PhoneNumber : INotifyPropertyChanged {

  public event PropertyChangedEventHandler PropertyChanged;

  private void OnPropertyChanged(string propertyName) {
    var handler = PropertyChanged;
    if (handler != null) {
      PropertyChanged(this, new PropertyChangedEventArgs(propertyName));
    }
  }

  private string m_CountryCode;
  public string CountryCode {
    get { return m_CountryCode; }
    set {
      m_CountryCode = value;
      ComputeFullNumber();
      OnPropertyChanged("CountryCode");
    }
  }

  private string m_AreaCode;
  public string AreaCode {
    get { return m_AreaCode; }
    set {
      m_AreaCode = value;
      ComputeFullNumber();
      OnPropertyChanged("AreaCode");
    }
  }

  private string m_Number;
  public string Number {
    get { return m_Number; }
    set {
      m_Number = value;
      ComputeFullNumber();
      OnPropertyChanged("Number");
    }
  }

  private string m_Suffix;
  public string Suffix {
    get { return m_Suffix; }
    set {
      m_Suffix = value;
      ComputeFullNumber();
      OnPropertyChanged("Suffix");
    }
  }

  public string FullNumber { get; private set; }

  private void ComputeFullNumber() {
    FullNumber = string.Format("+{0} ({1}) {2} - {3}", m_CountryCode, m_AreaCode, m_Number, m_Suffix);
    OnPropertyChanged("FullNumber");
  }

}

<DataTemplate x:Key="PhoneNumberTemplate">
  <StackPanel Orientation="Horizontal">
    <TextBox Text="{Binding Path=CountryCode, UpdateSourceTrigger=PropertyChanged}" />
    <TextBox Text="{Binding Path=AreaCode, UpdateSourceTrigger=PropertyChanged}" />
    <TextBox Text="{Binding Path=Number, UpdateSourceTrigger=PropertyChanged}" />
    <TextBox Text="{Binding Path=Suffix, UpdateSourceTrigger=PropertyChanged}" />
    <TextBox Text="{Binding Path=FullNumber, Mode=OneWay}" IsReadOnly="True" />
  </StackPanel>
</DataTemplate>

<TextBox Text="{Binding PhoneNumber, Converter={StaticResource PhoneNumberConverter, ConverterParameter=CountryCode}}"/>
<TextBox Text="{Binding PhoneNumber, Converter={StaticResource PhoneNumberConverter, ConverterParameter=AreaCode}}"/>
...