I have been looking and asking for information on this subject and getting nothing. I suspect that it is just a command in the data packet array. Any clues would be helpful. Can call $_REQUEST array be modified to inject needed code?
Leaving ringless voice mail drops with Twilio requires that you POST an api request to make a call single call to a phone number then a fraction of a second later make a second request to the same number while immediately dropping the first request. This results in the mobile company routing the second call directly to their voicemail servers.
As far as I know Twilio doesn't offer any "out of the box" voicemail drop options in their RESTful API.
来源：https://stackoverflow.com/questions/45948398/using-twilio-to-leave-ringless-voicemail-messages