sending sms message with vb and modem 3g - vb.net

Question

Hello guys I'm created a windows form that has a basic function of sending sms message to a phone by using my 3g modem. My modem 3g has load that will be use for texting. I check my device manager and I saw my modem 3g is already connected but Unfortunately, it's not sending sms message to my phone's number. here's my code so far. By the way my portname of device is com4
Imports System.IO.Ports
Imports System.Threading
Public Class Form1
Private Sub Form1_Load(sender As Object, e As EventArgs) Handles MyBase.Load
Try
Dim ports As String() = SerialPort.GetPortNames
Dim port As String
For Each port In ports
ComboBox1.Items.Add(port)
Next port
ComboBox1.SelectedIndex = 0
Catch ex As Exception
MsgBox(ex.Message)
End Try
End Sub
Private Sub Button1_Click(sender As Object, e As EventArgs) Handles Button1.Click
Try
With SerialPort1
.PortName = ComboBox1.Text
.BaudRate = 9600
.Parity = Parity.None
.StopBits = StopBits.One
.DataBits = 8
.Handshake = Handshake.RequestToSend
.DtrEnable = True
.RtsEnable = True
.NewLine = vbCrLf
.Open()
End With
Catch ex As Exception
MsgBox(ex.Message)
End Try
End Sub
Private Sub Button2_Click(sender As Object, e As EventArgs) Handles Button2.Click
Try
If SerialPort1.IsOpen Then
With SerialPort1
.Write("AT" & vbCrLf)
.Write("AT+CMGF=1" & vbCrLf)
.Write("AT+CMGS=" & Chr(34) & TextBox1.Text & Chr(34) & vbCrLf)
.Write(RichTextBox1.Text & Chr(26))
MsgBox("Succesfully Send!")
End With
Else
MsgBox("Port not open!")
End If
Catch ex As Exception
End Try
End Sub
End Class