I have a web service that gets a JSON request, makes a call and receives results that indicating user actions (pick up the call, drop the call).

What should I do in CallListener to send back this information ?

And how do I receive it in the web service asynchronously ?

Unfortunately, we don't have a good solution to this at the present time. We are investigating whether we can enable other JAX-RS implementations to be used with our next release. Some alternate implementations allow for asynchronous responses to be sent. With our 3.0 release, you'll have to handle things synchronously. The best practice would be to return a 200 or 204 immediately, then have the client poll for further status. You could use long polling if desired. If you're not expecting a large volume of requests, you could also use a lock to block the HTTP thread while waiting for subsequent callbacks.