'what api is used to make such call?

in the WebView app - a driver calls his rider without exposing his phone number -
a Twilio number already purchased and voice and sms already enabled -
when rider receives a call - he hears a text such as: this is your driver calling -
right after that he can answer/speak to his driver -
driver hears riders answering the call -

question: what api is used to make such call?

driver call rider

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