Verify phone number(Otp verification) using react-native-firebase

I am using react-native-firebase v^5.5.6. Now i want to just verify my phone number using firebase. To verify phone number i am using

firebase.auth().verifyPhoneNumber(phoneNumber).on('state_changed', (phoneAuthSnapshot) => {

switch (phoneAuthSnapshot.state) {

  case firebase.auth.PhoneAuthState.CODE_SENT: // or 'sent'
    console.log('code sent');

    break;
  case firebase.auth.PhoneAuthState.ERROR: // or 'error'
    console.log('verification error');

    break;

  // ---------------------
  // ANDROID ONLY EVENTS
  // ---------------------
  case firebase.auth.PhoneAuthState.AUTO_VERIFY_TIMEOUT: // or 'timeout'
    console.log('auto verify on android timed out');

    break;
  case firebase.auth.PhoneAuthState.AUTO_VERIFIED: // or 'verified'

    console.log('auto verified on android');
    console.log(phoneAuthSnapshot);

    break;
}
}, (error) => {

console.log(error);
}, (phoneAuthSnapshot) => {

console.log(phoneAuthSnapshot);
});

Using this method i got message state sent but always got code null

Response:

{verificationId: "AM5PThDkqMZC-alBDWvnn97ABxlVlFIZpEhN55sVLeR0b3_yar…rq9IksEpNBvU8tgv5kaE5CvWfIVv-Jn7YCNqVXtBrzD75j6og", code: null, error: null, state: "sent"}

Please help me.

Many thanks in advance