You call a number and Twilio says back "Hello World".
This demo is just one static XML file. Just post it on your web server (e.g. Apache), tell Twilio the URL of the file, and next time someone calls, Twilio will fetch the file and read back "Hello World". It's that simple!
For more information on how build applications that handle incoming calls take a look at the TwiML Quickstart.
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When a call is answered by Twilio, Twilio does an HTTP GET or POST for 'index.xml'