index.js

console.log('Loading function');
 
exports.handler = function(event, context) {
    var output ='<?xml version="1.0" encoding="UTF-8"?><Response><Pause length="2"/><Say>Hello Amazon, this is Twilio powered by Lambda</Say></Response>';
    context.succeed(output);
};

// Twilio sends requests with content type application/x-www-form-urlencoded
// To enable API gateway to parse them you need to go to Integration Request
// Under Integration Request go to Mapping Templates and set content type to
// application/x-www-form-urlencoded and select Mapping Template of type Empty
// Save your mapping template
// Screenshot of complete config: http://cl.ly/image/450F321o1X2O

// By default lambda will try to render the response as JSON which will break TwiML
// To override you need to go to Integration Response then Mapping Templates
// Create a new Mapping Template with the following code
// #set($inputRoot = $input.path('$'))
// $inputRoot
// Save this new mapping
// Screenshot of complete config: http://cl.ly/image/363O3p1V2s3t

// Lastly you need to make sure the respnse is returned in XML instead of JSON
// To configure this go to Method Response, expand the 200 tab and add a new
// Response Models for 200 entry. Set content type to application/xml and save
// Screenshot of complete config: http://cl.ly/image/1v1E3u2d3i1m