Angular Extended Service Test

[ALBy]

Hello, I am testing my app with Jasmine and Karma, but I am having some problems with my service. The code below is my service;

export class CustomerService extends SMService{

...
  getCustomers(sort: string,
               order: string,
               page: number,
               filter: CustomerFilter): Observable<SMServiceListResponse<Customer>> {
    return super.getObjects(sort, order, page, filter);
  }

...
}

I want to test if method 'getObjects' is called when getCustomers is executed. Below is the it() :

let mocker: Mocker;
let mockCustomer: any;
beforeEach(() => {
  mocker = new Mocker();
  mockCustomer = {data: mocker.getCustomers(2), pagination: {}};
});
afterEach(() => {
  httpTestingController.verify();
});

it('should return expected customers by calling once', () => {

  spyOn(customerService, 'getObjects').and.callThrough();
  customerService.getCustomers('', '', 0, null).subscribe(
      response => {
        expect(response).toEqual(mockCustomer, 'should return expected customers');
        expect(response.data.length).toEqual(mockCustomer.data.length);
      }
  );
  // Error: This is never called
  expect(customerService.getObjects).toHaveBeenCalledTimes(1);
  
  const req = httpTestingController.expectOne(customerEndpoint);
  expect(req.request.method).toEqual('GET');
  req.flush(mockCustomer); // Return expected Customers
});

Everything is ok but .getObjects is never called. I have tried a lot of ways to solve this but cant do it.