I have been playing with Twilio in the past weeks. It is pretty good and I have been able to send SMS from day one.
All good so far.
But I have failed miserably at placing calls.
Nothing showed in Twilio's debugger and it seemed as if nothing was even be triggered in Twilio at all.
Of course, I debugged my app (more details on that later) by pointing it to a mock Twilio service and it seemed to be doing everything it should. I have been pulling my hair over this issue for quite a while.
Sending SMS from my sandbox number, say +14156669999 to my cell, say +15127778888 works like a charm.... so why would placing a call from the same sandbox number to the same cell woud fail ? No idea. But that's just the way it is.
What you have to do is register one of your own numbers and initiate the call from the validated phone number to the cell number. The numbers I am talking about show in the Twilio account, on the "Phone Numbers" tab, in the section titled "My outgoing callerid numbers".
Unlike for SMS, you MUST use one of these numbers or Twilio just won't budge. Not even acknowledge that this is not what it expects....