[SOLVED] How to disable text-to-speech in verify user API in Nexmo?

And so I have this code:

const nexmo = require("./nexmo");

const requestId = null;

module.exports = function sendVerificationCode(
  recipient
) {
  return new Promise((resolve, reject) => {
    nexmo.verify.request({
        number: recipient,
        brand: 'Verification' 
    }, function(err, result) {
        if (result.status === 0) {
            requestId = result.request_id;
        }

        return requestId;
    });
  });
};

This successfully returns the request ID. But what I'm concerned about is I want the API's text-to-speech mechanism to be disabled. How?