Ennio, my understanding of calldata_type came from looking at its implementation in util/misc.py. Here’s the part I find the most interesting:
supported_call_data_types = ['string0', vtkCommonCore.VTK_STRING,
vtkCommonCore.VTK_OBJECT, vtkCommonCore.VTK_INT,
vtkCommonCore.VTK_LONG, vtkCommonCore.VTK_DOUBLE, vtkCommonCore.VTK_FLOAT]