write timeout error in vb.net to gsm modem send/rec sms apps sirs,

Imports System
Imports System.IO.Ports
Imports System.Threading

Public Class Form1

    Private Sub Form1_Load(ByVal sender As System.Object, ByVal e As System.EventArgs) Handles MyBase.Load
        Try

            Dim ports As String() = SerialPort.GetPortNames
            Dim port As String
            For Each port In ports
                CBCOMPORT.Items.Add(port)
            Next port
            CBCOMPORT.SelectedIndex = 0

        Catch ex As Exception
            MsgBox(ex.Message)
        End Try

    End Sub

    Private Sub BTNconnect_Click(ByVal sender As System.Object, ByVal e As System.EventArgs) Handles BTNconnect.Click
        Try
            With SerialPort1
                .PortName = CBCOMPORT.Text
                .BaudRate = 9600
                .Parity = Parity.None
                .StopBits = StopBits.One
                .Handshake = Handshake.RequestToSend
                .DataBits = 8
                .DtrEnable = True
                .RtsEnable = True
                .NewLine = vbCrLf
                .Open()
            End With
        Catch ex As Exception
            MsgBox(ex.Message)

        End Try

    End Sub

    Private Sub Btnsend_Click(ByVal sender As System.Object, ByVal e As System.EventArgs) Handles BTNSEND.Click
        Try

            If SerialPort1.IsOpen Then
                With SerialPort1

                    .Write("AT" & vbCrLf)
                    .Write("AT+CMGF=1" & vbCrLf)
                    .Write("AT+CMGS=" & Chr(34) & TBCPNUM.Text & Chr(34) & vbCrLf)
                    .Write(RTBMESSAGE.Text & Chr(26))
                    MsgBox("SENT MESSAGE")

                End With
            End If

        Catch ex As Exception
            MsgBox(ex.Message)
        End Try
      
    End Sub
End Class