<Hangup>The <Hangup> verb ends a call. If used as the first verb in a TwiML response
it does not prevent Twilio from answering the call and billing your
account. The only way to not answer a call and prevent billing is to use the
<Reject> verb.
The <Hangup> verb has no attributes.
You can't nest any verbs within <Hangup> and you can't nest <Hangup> within any other verbs.
The following code tells Twilio to answer the call and hang up immediately.
<?xml version="1.0" encoding="UTF-8"?>
<Response>
<Hangup/>
</Response>
<Gather>, <Record>, <Dial> or <Sms>, you can return a response containing the <Hangup> verb to end the current call.