A request to make a phone call may take a "long" time (in terms of web requests that means "several seconds"). Therefore the API cannot answer with a single return code, but it will update your status after some "some time" using a post-back URL. Try providing a URL to your own app and see what comes back.
Trouble is in order to do this you will most probably need a deployed app as you need a publicly available URL to do so (and localhost
will not do :-). So you will have to setup a test domain, but most probably you will be able to use a subdomain, something like beta.mydomain.com
in order to figure out what you will get.
Most of the examples they show on their website seem to be in PHP, but the callbacks they provide after making a call seem to expect to be called after "some time" if a connection could be established.
The API-Link you provided states rather clearly
After a call ends, Twilio will make an asynchronous HTTP request to the StatusCallback URL if you provided one in your POST. This will happen regardless of the call status.
so that is what you should expect, an asynchronous HTTP request
to the callback-link you provide. This is made asynchronously as Twillio can not maintain an http-connection for the (indefinite) duration of your call. So you should provide it with an URL
such as
http://beta.mydomain.com/call-me-back-here
and wire it to an appropriate route (most probably a POST
-request).