Codenstuff.com

    Dim log As String = ""
    Dim stage As Integer = 0

    Dim number As String = "00000000000"
    Dim message As String = "SMS Test"

    Sub sp_DataReceived(ByVal sender As SerialPort, ByVal e As SerialDataReceivedEventArgs)
        Dim buffer(sender.BytesToRead - 1) As Byte
        sender.Read(buffer, 0, buffer.Length)

        Dim rec As String = Encoding.ASCII.GetString(buffer)
        log += rec

        'MsgBox(log) 'debug

        If log.EndsWith("ERROR" & vbCrLf) Then
            sender.Close()
            MsgBox("Error at stage " & stage)
            MsgBox(log)
            stage = 0
            log = ""

        ElseIf log.EndsWith("OK" & vbCrLf) Then

            If stage = 0 Then
                log = ""
                stage += 1

                sender.Write("AT+CMGF=1" & vbCrLf) ' set to text mode

            ElseIf stage = 1 Then
                log = ""
                stage += 1

                sender.Write("AT+CMGS=""" & number & """" & vbCrLf) 'set recipient number, await message input

            ElseIf stage = 3 Then
                sender.Close()
                MsgBox("Message sent. Status: " & log)
                log = ""
                stage = 0
            End If

        End If

        If stage = 2 And log.EndsWith("> ") Then
            log = ""
            stage += 1

            sender.Write(message & Chr(26)) 'CTRL+Z
        End If
    End Sub

    Private Sub btnSend_Click(ByVal sender As System.Object, ByVal e As System.EventArgs) Handles btnSend.Click

        Dim sp As SerialPort = New SerialPort("COM5") 'use the COM port of your modem device

        Try
            sp.Open()
        Catch ex As Exception
            MsgBox("Error opening port: " & ex.Message)
            Return
        End Try

        AddHandler sp.DataReceived, AddressOf sp_DataReceived
        sp.Write("AT" & vbCrLf) 'OK expected
    End Sub