.NET (Core and Framework)

        static void ReadExcel(String strFileName, string strDelimiter) 
        {
            int RowCount = 0, ColCount = 0;

            //Write to text
            StreamWriter sw = new StreamWriter(@"C:\Test Files\Test.txt");
            String strCell = "";
            int col;

            Object Opt = Missing.Value;
            Microsoft.Office.Interop.Excel.Application app = new Microsoft.Office.Interop.Excel.Application(); //= new Excel.Application();
                                                                                                               //Declare Workbook
            Microsoft.Office.Interop.Excel.Workbook book;

            //Excel.Worksheet sheet;
            Microsoft.Office.Interop.Excel.Worksheet xlsSheet;
            Microsoft.Office.Interop.Excel.Range oRng;

            //Open Spreadsheet
            book = app.Workbooks.Open(strFileName, Opt, Opt, Opt, Opt, Opt, Opt, Opt, Opt, Opt, Opt, Opt, Opt, Opt, Opt);

            //Take the first sheet
            xlsSheet = (Microsoft.Office.Interop.Excel.Worksheet)book.Sheets[1];

            if (xlsSheet != null)
            {
                //Read the first cell
                oRng = (Microsoft.Office.Interop.Excel.Range)xlsSheet.Cells[1, 1];

                ColCount = xlsSheet.Columns.Count;

                RowCount = xlsSheet.Rows.Count;
            }

            for (int row = 0; row < RowCount; row++)
            {
                strCell = "";
                for (col = 0; col < ColCount; col++)
                {
                    oRng = (Microsoft.Office.Interop.Excel.Range)xlsSheet.Cells[row + 1, col + 1];
                    if (col < ColCount - 1)
                    {
                        strCell = strCell + oRng.Text.ToString() + strDelimiter;
                    }
                    else
                    {
                        strCell = strCell + oRng.Text.ToString();
                    }
                }
                //Write to text file
                sw.WriteLine(strCell);
            }

            Console.WriteLine(strCell);

            sw.Close();

            book.Close(false, false, Missing.Value);
        }

Dim Number1 As String = "2812"
Dim Str, strTemp, strReturn As String
Dim x As Long

Str = "ATDT" & Number1 & "R"  ' ATDT2812R

For x = 1 To Len(Str)
    strTemp = Hex$(Asc(Mid$(Str, x, 1)))
    If Len(strTemp) = 1 Then strTemp = "0" & strTemp & ","
    strReturn = strReturn & Space$(1) + "&H" & strTemp
 Next x

   strReturn = stReturn & " &HD"  '  &H41, &H54, &H44, &H54, &H32, &H38, &H31, &H32, &H52, &HD

'  Now I want to write this to Serialport1

'  ATDT + NUMBER1 + R. = &H41, &H54, &H44, &H54, &H32, &H38, &H31, &H32, &H52, &HD

Dim Num as integer = Convert.ToInt32(strReturn)

buffer = {Num}
frmMain.SerialPort1.Write(buffer, 0, buffer.Length)
Threading.Thread.Sleep(500)

'  This Doesn't Work But If I hard code Number1

buffer = {&H41, &H54, &H44, &H54, &H32, &H38, &H31, &H32, &H52, &HD}
frmMain.SerialPort1.Write(buffer, 0, buffer.Length)
Threading.Thread.Sleep(500)
Next x

'  Then It Works No problem
'  Can Someone Help With This - Thanks