First, a non-programming question: My phone is auto-responding to missed calls with a text message: "Sorry. I'm not near my phone right now." Where did my phone get the idea to do that and how do I make it stop?
And a programming question: How can I make my Android do arbitrary things in response to a missed call, such as email me the phone number, or just customize the SMS auto-response, perhaps replying with my location.
The answer to your programming question:
Setup an IntentListener for incoming calls. If it's not answered, have your application process it accordingly!