Using Chr in a Windows Service

Hello everyone, first timer here.

I've been working on this problem all day and still can't get it to work.

Situation:  I am writing a Windows Service that will communicate with a mobile phone, connected via USB, in order to send a text message.

Problem:  I'm using AT commands in order to achieve this, however when I send the chr(26) after the message (ctrl+z to send the message), it doesn't do anything.  I have pasted the same code into a windows form and it works fine.  Is there a limitation of windows services that I don't know about?

Code: Dim number As String = "+447824123456"
          Dim message As String = TextBox1.Text
          Dim serialport As New IO.Ports.SerialPort

          With serialPort
              .PortName = "COM24"
              .BaudRate = "9600"
              .Parity = IO.Ports.Parity.None
              .DataBits = 8
              .StopBits = IO.Ports.StopBits.One
              .Handshake = IO.Ports.Handshake.RequestToSend
              .DtrEnable = True
              .RtsEnable = True
          End With

          serialPort.Open()
          'checks phone status
          serialPort.WriteLine("AT" & vbCrLf)
          'Configures message as SMS
          serialPort.WriteLine("AT+CMGF=1" & vbCrLf)
          'Sets message center number
          serialPort.WriteLine("AT+CSCA=""+447785016005""" & vbCrLf)
          'Sets destination number
          serialPort.WriteLine("AT+CMGS=""" & number & """" & vbCrLf)
          'Specifies message and sends Ctrl+z
          serialPort.WriteLine(message & Chr(26))
          'Displays buffer containing output messages
          System.Threading.Thread.CurrentThread.Sleep(2000)
          MsgBox(serialPort.ReadExisting)
          serialPort.Close()

Thanks for your help in advance.

James