问使用GSM手机的AT命令发送的SMS出现错误

EN

        SmsFields smsObj = null;
        List<SmsFields> smsColl = null;
        SerialPort serialport = null;
        StringBuilder strbuild = new StringBuilder();
        try
        {
            //Validate the form 
            if (!Validation()) return;

            serialport = new SerialPort();

            ////Sets the properties of serial port object
            serialport.PortName = cboPort.SelectedItem.ToString();
            serialport.BaudRate = 9600;
            serialport.Parity = Parity.None;
            serialport.DataBits = 8;
            serialport.StopBits = StopBits.One;
            serialport.Handshake = Handshake.RequestToSend;
            serialport.DtrEnable = true;
            serialport.RtsEnable = true;

            //Open the port to send sms
            serialport.Open();

            //Check if port is opened or not
            if (!serialport.IsOpen)
            {
                MessageBox.Show("Serial port is not opened. Please try with other port");
                return;
            }

            //Create smsFields class's object and fill the data in the generic collection
            smsObj = SmsFields.Instance;
            smsColl = smsObj.FillData(txtFilePath.Text);

            if (smsColl == null)
            {
                MessageBox.Show("No data found in the excel table");
                return;
            }
            //Gets the single record from SmsFields class and sends the message
            foreach (SmsFields sms in smsColl)
            {

                //checks phone status
                serialport.WriteLine("AT" + Environment.NewLine);
                //Configures message as SMS (0 for PDU format) and (1 for text format)
                serialport.WriteLine("AT+CMGF=1" + Environment.NewLine);

                //Sets message center number
                serialport.WriteLine("AT+CSCA=\"" + txtServiceNo.Text + "\"" + Environment.NewLine);

                //Sets destination number
                serialport.WriteLine("AT+CMGS=\"" + sms.DestinationNo + "\"" + Environment.NewLine);

                //Specifies message and sends Ctrl+z
                serialport.WriteLine(sms.Message + (char)26);

                //Displays buffer containing output messages
                System.Threading.Thread.Sleep(4000);
   }