Need help for may sms gateway

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VB

SMS

i try to send sms by modem prolink phs301 using AT command, port has been detected on my combo box but when i click send button there is nothing happened. sms not deliver to my number
this is my script

What I have tried:

Imports System.IO.Ports
Imports System.Threading.Thread
Public Class Form1
    Private Tunda As Integer
    Private WithEvents COMport As New SerialPort
    Private Sub Form1_Load(ByVal sender As Object, ByVal e As System.EventArgs) Handles Me.Load
        mati()
        For Each COMString As String In My.Computer.Ports.SerialPortNames
            cmbKoneksi.Items.Add(COMString)
        Next
        cmbKoneksi.Sorted = True
    End Sub
    Private Sub Button1_Click(ByVal sender As System.Object, ByVal e As System.EventArgs) Handles btnKirim.Click
        If cmbKoneksi.Text = "" Or tnohp.Text = "" Or tPesan.Text = "" Then
            MsgBox("Silahkan Isi Semua")
        Else
            COMport.PortName = cmbKoneksi.Text
            COMport.BaudRate = 9600
            COMport.WriteTimeout = 300
            Try
                COMport.Open()
                bersih()
            Catch ex As Exception
                MsgBox(ex.Message)
            End Try
            ' Sleep(300)   '....tunggu 0.3 second
            Tunda = 300
            Sleep(Tunda)
            Application.DoEvents()
            If COMport.IsOpen Then
                Try
                    COMport.WriteLine("AT+CMGF=1" & Chr(13))
                    Sleep(Tunda)
                    COMport.WriteLine("AT+CMGS=" & Chr(34) & tnohp.Text & Chr(34) & Chr(13))
                    Sleep(Tunda)
                    COMport.WriteLine(tPesan.Text & Chr(26))
                    Sleep(Tunda)
                Catch ex As Exception
                    MsgBox(ex.Message)
                End Try
            Else
                MsgBox("COM port tertutup.")
            End If
            COMport.Close()
        End If

    End Sub
    Sub bersih()
        cmbKoneksi.Text = ""
        tnohp.Clear()
        tPesan.Clear()
        mati()
    End Sub
    Sub mati()
        cmbKoneksi.Enabled = False
        tnohp.Enabled = False
        tPesan.Enabled = False
        btnKirim.Enabled = False
        btnTulisBaru.Enabled = True
    End Sub
    Sub hidup()
        cmbKoneksi.Enabled = True
        tnohp.Enabled = True
        tPesan.Enabled = True
        btnKirim.Enabled = True
        btnTulisBaru.Enabled = False
    End Sub
    Private Sub btnTulisBaru_Click(ByVal sender As Object, ByVal e As EventArgs) Handles btnTulisBaru.Click
        hidup()
    End Sub
End Class