detect if private static final field's method is called with mockito

I want to test onFailure method in my class and detect if post method is called on BUS instance.
I've been trying all day and I couldn't have accomplish.
I use powermock and Mockito with junit.

ApiCallback.java  


package com.example.android.webserver;

import com.example.android.event.OnApiRequestErrorEvent;
import com.example.android.event.OnApiResponseErrorEvent;
import com.example.android.model.ApiRequestError;
import com.example.android.utils.BusProvider;
import com.example.android.utils.ErrorUtils;
import org.greenrobot.eventbus.EventBus;
import retrofit2.Call;
import retrofit2.Callback;
import retrofit2.Response;

public abstract class ApiCallback<T> implements Callback<T> {
private final static EventBus BUS;

static {
    BUS = BusProvider.getInstance();
}


@Override
public void onFailure(Call<T> call, Throwable t) {
    BusProvider.getInstance().post(new OnApiRequestErrorEvent(new ApiRequestError(t.getMessage())));
}


protected void handleResponse(Response<?> response, Object object) {
    if (response.isSuccessful()) {
        BusProvider.getInstance().post(object);
    } else {
        BUS.post(new OnApiResponseErrorEvent(ErrorUtils.parseError(response), object));
    }
}
}

My test class:

ApiCallbackTest.java


package com.ashojash.android.webserver;

import com.ashojash.android.utils.BusProvider;
import org.greenrobot.eventbus.EventBus;
import org.junit.Before;
import org.junit.Test;
import org.junit.runner.RunWith;
import org.mockito.Mock;
import org.powermock.api.mockito.PowerMockito;

import static org.mockito.Mockito.verify;
import static org.powermock.api.mockito.PowerMockito.*;
import org.powermock.core.classloader.annotations.PrepareForTest;
import org.powermock.modules.junit4.PowerMockRunner;
import org.powermock.reflect.Whitebox;
import retrofit2.Call;

import static org.mockito.Matchers.any;
import static org.mockito.Matchers.anyObject;
import static org.mockito.Mockito.doCallRealMethod;
import static org.powermock.api.mockito.PowerMockito.mockStatic;


@RunWith(PowerMockRunner.class)
@PrepareForTest({BusProvider.class})
public class ApiCallbackTest {

@Mock
private ApiCallback apiCallback;
@Mock
private Call call;
@Mock
private Throwable throwable;

@Before
public void setUp() {
}

@Test
public void should_call_busProvider_to_post_event() throws Exception {
//        given
    doCallRealMethod().when(apiCallback).onFailure(any(Call.class),   any(Throwable.class));
    doCallRealMethod().when(BusProvider.getInstance());
//        when
    apiCallback.onFailure(call, throwable);

//        then
    EventBus bus = BusProvider.getInstace();
    verify(bus).post(anyObject());
 }
}