You can use the Windows API function CreateFile() to get a handle to the com port, and use standard file I/O to communicate with the given port. Example:

var
  hCommFile : THandle;
procedure TForm .Button Click(Sender: TObject);
var
  PhoneNumber : string;
  CommPort : string;
  NumberWritten : LongInt;
begin
  PhoneNumber := 'ATDT  -555-555- 2 2' + # 3 + # 0;
  CommPort := 'COM2';
 {Open the comm port}
  hCommFile := CreateFile(PChar(CommPort),
                          GENERIC_WRITE,
                          0,
                          nil,
                          OPEN_EXISTING,
                          FILE_ATTRIBUTE_NORMAL,
                          0);
  if hCommFile=INVALID_HANDLE_VALUE then
  begin
    ShowMessage('Unable to open '+ CommPort);
    exit;
  end;
 {Dial the phone}
  NumberWritten:=0;
  if WriteFile(hCommFile,
               PChar(PhoneNumber)^,
               Length(PhoneNumber),
               NumberWritten,
              nil) = false then begin
    ShowMessage('Unable to write to ' + CommPort);
  end;
end;
procedure TForm .Button2Click(Sender: TObject);
begin
 {Close the port}
  CloseHandle(hCommFile);
end;