This function worked for me. It sends a PDU message.

Private Sub PDUSMS()
If Comport = "" Then MsgBox("Please Choose A connection") : Return
smsPort.PortName = Comport
smsPort.BaudRate = "115200"
smsPort.Parity = Parity.None
smsPort.DataBits = 8
smsPort.StopBits = StopBits.One
smsPort.Handshake = Handshake.RequestToSend
smsPort.DtrEnable = True
smsPort.RtsEnable = True
smsPort.NewLine = vbCrLf

    If Not (smsPort.IsOpen = True) Then
        smsPort.Open()
    End If
    At\_length = "17"
    PDU\_code = "079144772800800011000C914477529628010000AA03E8721E"
    'smsPort.WriteLine("AT" & vbCrLf)
    'System.Threading.Thread.Sleep(2000) 'Pause to allow last command to get to phone
    smsPort.WriteLine("AT+CMGF=0" & vbCrLf) 'open space for new message
    System.Threading.Thread.Sleep(2000) 'Pause to allow last command to get to phone
    smsPort.WriteLine("AT+CSMS=0" & vbCrLf) 'open space for new message
    System.Threading.Thread.Sleep(2000) 'Pause to allow last command to get to phone
    smsPort.WriteLine("AT+CMGS=" + At\_length + vbCrLf) '+ Chr(34) + At\_length + Chr(34) +
    System.Threading.Thread.Sleep(2000) 'Pause to allow last command to get to phone
    smsPort.WriteLine(PDU\_code + Chr(26)) 'number to send message to  + vbCrLf +
    System.Threading.Thread.Sleep(2000)

    If smsPort.IsOpen = True Then
        smsPort.Close()
    End If
End Sub