Twilio Evangelist here.
So, I actually took a copy of the above and tried it with my own account/numbers/URLs, and everything seemed fine. I would suggest you use your call logs to look into it. If you open up one of your test calls, have a look at the Twilio Request Inspector, this should allow you to see what is going on, or if you are getting any errors.
As I say, it should work fine, and I had the same thing working here.
You may want to contact Twilio support on help@twilio.com.
Good luck!