Unfortunately, the Twilio Client SDK requires all client-side apps to have a custom server-side counterpart that issues your app capability tokens and hosts TwiML to handle the calls. To hopefully make your life easier, I've created the repo above to address the server-side needs of the most common use cases for Twilio Client. It can be easily deployed to your heroku account in about a minute – simply follow the readme on that repo to get the server set up.