ObservableCollection<T> not updating UI

public class MessageList : ObservableCollection<LobbyMessage>
{
    public MessageList(): base()
    {
        Add(new LobbyMessage() { Name = "System", Message = "Welcome!" });
    }
}

static public MessageList LobbyMessages { get; set; }

ChatHelper.LobbyMessages = new MessageList();

<ListBox IsSynchronizedWithCurrentItem="True"
         ItemsSource="{Binding Mode=OneWay}"
         x:Name="ListBoxChatMessages"
         d:UseSampleData="True"
         ItemTemplate="{DynamicResource MessageListTemplate}"
         IsEnabled="True">
      <ListBox.DataContext>
    	<Magrathea_Words_Tools:MessageList/>
       </ListBox.DataContext>
    </ListBox>

Helper.Context = SynchronizationContext.Current;

#region IServiceMessageCallback Members

/// <summary>
/// Callback handler for when the service has a message for
/// this client
/// </summary>
/// <param name="serviceMessage"></param>
public void OnReceivedServiceMessage(ServiceMessage serviceMessage)
{
    // This is being called from the WCF service on it's own thread so
    //  we have to marshall the call back to this thread.
    SendOrPostCallback callback = delegate
    {
        switch (serviceMessage.MessageType)
        {
            case MessageType.ChatMessage:
                ChatHelper.LobbyMessages.Add(
                        new LobbyMessage()
                        {
                            Name = serviceMessage.OriginatingPlayer.Name,
                            Message = serviceMessage.Message
                        });
                break;

            default:
                break;
        }
    };

    Helper.Context.Post(callback, null);
}

ListBoxChatMessages.ItemsSource = ChatHelper.LobbyMessages.Messages;

<ListBox IsSynchronizedWithCurrentItem="True"
       ItemsSource="{Binding Mode=OneWay}" Background="#FF1F1F1F"
       Margin="223,18.084,15.957,67.787" x:Name="ListBoxChatMessages"
       ItemTemplate="{DynamicResource MessageListTemplate}"
       IsEnabled="True"/>