When I am trying to execute answering machine Devices column getting blank ,TAPIMEDIATYPE_AUDIO=0

The Media_Modes property constants describe media types (or modes)of a communications session or call. You modem Media_Modes And TAPIMEDIATYPE_AUDIO=0 mean either your modem do NOT support voice function or your modem 's wave serial driver was not installed correctly. Make sure your modem did support voice function. If your modem did support voice function, try to reinstall the driver or install the latest driver.
You can get your modem 's capabilities through the DeviceCaps demo.