Sending SMS via USB GSM Modem - vb.net

Question

I am trying to develop a module to send SMS from my application. The problem is that when I send an AT Command to a connected cell phone, my app stops responding. I'm using a Nokia 6720C and I have installed Pc Suite. It also appears in the Com Ports.
My code is below. What am I doing wrong?
Imports System
Imports System.IO.Ports
Imports System.Threading
Imports System.ComponentModel
Public Class Form1
Delegate Sub SetTextCallback(ByVal [text] As String)
Private Sub btnSend_Click(ByVal sender As System.Object, ByVal e As System.EventArgs) Handles btnSend.Click
With SerialPort1
.PortName = "COM14"
.DataBits = 8
.Parity = IO.Ports.Parity.None
.StopBits = StopBits.One
End With
SerialPort1.Open()
SerialPort1.Write("AT" & vbCr)
SerialPort1.Close()
End Sub
Private Sub SerialPort1_DataReceived(ByVal sender As Object, ByVal e As System.IO.Ports.SerialDataReceivedEventArgs) Handles SerialPort1.DataReceived
ReceivedText(SerialPort1.ReadExisting())
End Sub
Private Sub ReceivedText(ByVal [text] As String)
'compares the ID of the creating Thread to the ID of the calling Thread
If Me.TxtResponse.InvokeRequired Then
Dim x As New SetTextCallback(AddressOf ReceivedText)
Me.Invoke(x, New Object() {(text)})
Else
Me.TxtResponse.Text &= [text]
End If
End Sub

Answer 1

Well, at the end of the AT command you must add Chr(13) and Chr(10), you used Chr(13) only (which is "vbCr"), try replacing it with vbCrLf (Carriage-Return "Chr(13)" and Line Feed "Chr(10)"). I think this is the problem (the command string is not complete).
If the problem still resides, try the following to test your work:
Try using your VB.NET code to communicate with a GSM modem (not your phone). If it works fine, then maybe the phone does not accept AT commands.
Try using a terminal program to send the AT command to the phone. The terminal program can be like "Hyper Terminal" or "Maestro Smart Terminal" (you can search for it on google).
I hope this was helpful, good luck.