Passworless (SMS) login

Hi everyone,
I’m trying to create an embedded passwordless login in my react native app following this guide:

I’m using SMS codes.
I’m able to receive a code in my phone but when i send back a post request to validate it I get this error:

error: "invalid_grant"
error_description: "Wrong phone number or verification code."

This is code I use:

var axios = require("axios").default;

var options = {
  method: 'POST',
  url: 'https://{yourDomain}/oauth/token',
  headers: {'content-type': 'application/json'},
  data: {
    grant_type: '',
    client_id: '{yourClientId}',
    username: 'USER_PHONE_NUMBER',
    otp: 'code',     // I placed here the code received
    realm: 'sms',
    audience: 'your-api-audience',
    scope: 'openid profile email'

axios.request(options).then(function (response) {
}).catch(function (error) {

My problem is that It’s not clear where should I put the code received. I placed the CODE received in the “opt” attribute.
Also, why it’s also asking for a username?.

thanks in advanced for your help.