دریافت پیام delivary با gsmcomm

سلام من برای دریافت پیغام delivary report در رویداد message recevied کد زیر را نوشتم ولی با پیغام message service error 321 در دستور DecodedShortMessage msg = comm.ReadMessage(loc.Index, loc.Storage) مواجه شدم

private void comm_MessageReceived(object sender, GsmComm.GsmCommunication.MessageReceivedEventArgs e)
{
try
{
IMessageIndicationObject obj = e.IndicationObject;            
//Get status report for this condition
if (obj is MemoryLocation) 
{
MemoryLocation loc = (MemoryLocation)obj;                                                    
Output(string.Format("New message received in storage "{0}", index {1}.",
loc.Storage, loc.Index));                
Output("");
DecodedShortMessage msg = comm.ReadMessage(loc.Index, loc.Storage);
if (((SmsPdu)msg.Data) is SmsStatusReportPdu)
{
SmsStatusReportPdu data = (SmsStatusReportPdu)msg.Data;
Output("rec msg ref #: " + data.MessageReference);
}
Output(msg.Status.ToString());
}
}
catch (Exception ex)
{
MessageBox.Show(ex.Message);
}