UDP multicast group on Windows Phone 8

OK this is one I've been trying to figure out for a few days now. We have an application on Windows Phone 7 where phones join a multicast group and then send and receive messages to the group to talk to each other. Note - this is phone to phone communication.

Now I'm trying to port this application to Windows Phone 8 - using the 'Convert to Phone 8' feature in Visual Studio 2012 - so far so good. Until I try to test the phone to phone communication. The handsets seem to join the group fine, and they send the datagrams OK. They even receive the messages that they send to the group - however, no handset ever receives a message from another handset.

Here is the sample code behind to my page:

// Constructor
public MainPage()
{
    InitializeComponent();
}

// The address of the multicast group to join.
// Must be in the range from 224.0.0.0 to 239.255.255.255
private const string GROUP_ADDRESS = "224.0.1.1";

// The port over which to communicate to the multicast group
private const int GROUP_PORT = 55562;

// A client receiver for multicast traffic from any source
UdpAnySourceMulticastClient _client = null;

// Buffer for incoming data
private byte[] _receiveBuffer;

// Maximum size of a message in this communication
private const int MAX_MESSAGE_SIZE = 512;

private void PhoneApplicationPage_Loaded(object sender, RoutedEventArgs e)
{
    _client = new UdpAnySourceMulticastClient(IPAddress.Parse(GROUP_ADDRESS), GROUP_PORT);
    _receiveBuffer = new byte[MAX_MESSAGE_SIZE];

    _client.BeginJoinGroup(
        result =>
        {
            _client.EndJoinGroup(result);
            _client.MulticastLoopback = true;
            Receive();
        }, null);
}

private void SendRequest(string s)
{
    if (string.IsNullOrWhiteSpace(s)) return;

    byte[] requestData = Encoding.UTF8.GetBytes(s);

    _client.BeginSendToGroup(requestData, 0, requestData.Length,
        result =>
        {
            _client.EndSendToGroup(result);