public interface IEvents<T>
{
  [OperationContract(IsOneWay = true)]
  AddEvent(T newItem);

  [OperationContract(IsOneWay = true)]
  DeleteEvent(int idDeletedItem);
}

[ServiceContract(CallbackContract = typeof(IEvents<User>))]
public interface IUsers
{
  [OperationContract(IsOneWay = true)]
  void AddUser(string login, string fio);

  [OperationContract(IsOneWay = true)]
  void DeleteUser(int id);

  [OperationContract(IsOneWay = true)]
  void Subscribe();

  [OperationContract(IsOneWay = true)]
  void UnSubscribe();

}

[ServiceContract(CallbackContract = typeof(IEvents<Department>))]
public interface IDepartments
{
  [OperationContract(IsOneWay = true)]
  void AddDepartment(string name, int state);

  [OperationContract(IsOneWay = true)]
  void DeleteDepartment(int id);

  [OperationContract(IsOneWay = true)]
  void ChangeState(int id, int state);

  [OperationContract(IsOneWay = true)]
  void Subscribe();

  [OperationContract(IsOneWay = true)]
  void UnSubscribe();

}

public class Service: IUsers, IDepartments
{
  .....
}