Sending SMS using usb dongle

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C#

I created a web form to send sms using a usb dongle. it worked properly at the first time and then it didn't work; But after several time it worked again. Whats wrong with this. I have attach my code follow.

What I have tried:

int mSpeed = 1;
                serialPort.PortName = "COM6";
                serialPort.BaudRate = 9600;
                serialPort.Parity = Parity.None;
                serialPort.DataBits = 8;
                serialPort.StopBits = StopBits.One;
                serialPort.Handshake = Handshake.XOnXOff;
                serialPort.DtrEnable = true;
                serialPort.RtsEnable = true;
                serialPort.NewLine = Environment.NewLine;
                //serialPort.BreakState = true;
                serialPort.Open();

                serialPort.WriteLine("AT+CMGF=1" + Environment.NewLine);
                System.Threading.Thread.Sleep(200);
                serialPort.WriteLine("AT+CSCS=GSM" + Environment.NewLine);
                System.Threading.Thread.Sleep(200);
                serialPort.WriteLine("AT+CMGS=" + (char)34 + lblphone.Text + (char)34 + Environment.NewLine);    //Set Recievers Phone Number
                System.Threading.Thread.Sleep(200);
                serialPort.WriteLine(txtMessage.Text + (char)26);
                System.Threading.Thread.Sleep(mSpeed);
                serialPort.Close();