As you've had two amusing answers, perhaps I'll try and help you a bit.

The short answer is you probably can't.

The issue you will run into is with the telco involved. It is unlikely that you will be able to detect an answering machine, because very few actually play anything back up the line in a machine-readable format. Some of those available in the UK could be accessed remotely for playback of messages; however there is a privacy issue there. The issue is similar to that of receiving party CLI being sent by the telco. That's why you don't often get the callers number displayed on your 'phone when it's an international caller. There just aren't standards and in many countries displaying a full number is illegal anyway (France for example).

Can you explain what you're trying to do?

It could be that there are toolkits out there that can do this already and then you interface with that through COM. I used to deal with a company called HTK in Ipswich, UK that could do this sort of thing (www.htk.co.uk)

Regards
Simon