Firebase not sending verification code to phone the second time

If you read Firebase Docs about PhoneAuthProvider, it is clearly stated:

The verifyPhoneNumber method is reentrant: if you call it multiple times, such as in an activity's onStart method, the verifyPhoneNumber method will not send a second SMS unless the original request has timed out.

In your case, you set the timeout at 120 seconds. If you really want to test this behavior, I suggest reducing the timeout to 10 or 20 seconds, and see if you can resend OTP after the timeout.

Additionally, if you want to get the timeout event, you can add onCodeAutoRetrievalTimeOut as a part of PhoneAuthProvider.OnVerificationStateChangedCallbacks. I think it is safe to set your resendOtp enabled as true in that callback. And it is way better than defining your own timer.

You just try to Logout from your app or one more thing you can do is to clear app data or you can reinstall.

@Override
public void onVerificationCompleted(PhoneAuthCredential credential) {
    // This callback will be invoked in two situations:
    // 1 - Instant verification. In some cases the phone number can be instantly
    //     verified without needing to send or enter a verification code.
    // 2 - Auto-retrieval. On some devices Google Play services can automatically
    //     detect the incoming verification SMS and perform verification without
    //     user action.
    Log.d(TAG, "onVerificationCompleted:" + credential);

    String code = phoneAuthCredential.getSmsCode();

        Log.d("REGISTER_USER", "code generated first " + code);
        if (code != null) {
            //verifying the code like in normal flow
            verifyUserAuthCode(code);
        } else {
           //you dont get any code, it is instant verification
            showProgressBar(WAIT_TITLE, VERIFYING_MESSAGE);
            signInWithPhoneAuthCredential(phoneAuthCredential);
        }
}
    private void signInWithPhoneAuthCredential(PhoneAuthCredential credential) {
          auth.signInWithCredential(credential)
            .addOnCompleteListener(this, new OnCompleteListener<AuthResult>() {
                @Override
                public void onComplete(@NonNull Task<AuthResult> task) {
                    final FirebaseUser user = auth.getCurrentUser();

                    if (task.isSuccessful()) {//user verified}

            });
}

Read above comments in official Google docs carefully, firebase uses Instant verification without sending phone number, for this you just need to validate "credentials" object , write logs you will find that this method will be executed everytime you send the verification code request

It's important to keep in mind that, phone numbers that you have added to "Phone Numbers for Testing" in Authentication section in Firebase console, as shown here ,will not be expected to receive any SMS OTP code. Instead, you are expected to enter the code you have already freely picked there on console.

Further situation in which "No OTP SMS is expected to receive":

When you repetitively use a specific phone number, signing in and out for the sake of testing, without adding it to "Phone Numbers for Testing", Google might eventually block it temporarily with an error indicating unusual behavior has been detected, or with error: "SMS verification code request failed: unknown status code: 17010 null".