C++11 Polymorphism : Call derived class method from base class pointer

In summary, I believe that the answer to my question is no. However, I will have hope and defer to those smarter than I.

I would like to call a class method of a derived class from the pointer to the base class. The inherited method(s) that are overloaded and desired to be called are pure virtual. I am able to get it to work in certain cases but not in one particular case, where the pure virtual method contains ellipsis placeholder for arguments.

The essence of what I desire is pasted below, any fixes and/or suggestions are greatly appreciated, thank you.

    #include <iostream>

    class A
    {
        public:
            virtual void setup()=0;
            virtual void callFunction(std::string str, ...)=0;
    };

    class B: public A
    {
        public:
            virtual void setup() {};        
            virtual void callFunction(std::string str, ...) { std::cout<<"\nB::callFunction("<<str<<")\n"; }; //compiles: not desired: a->callFunction("Hello World", 987); executes as B::callFunction
            //virtual void callFunction(std::string str, ...)=0; compile error: error: cannot declare variable 'c' to be of abstract type 'C'
    };

    class C: public B
    {
        public:
            void setup() { std::cout<<"\nC::setup\n"; };
            void callFunction(std::string str, int i) { std::cout<<"\nC::callFunction("<<str<<") = "<<i<<"\n"; };
    };

    int main()
    {          
        C c;
        A* a = (A*) new C;
        
        c.setup();
        c.callFunction("Hello World", 123);
        
        a->setup(); //WHY: does this pure virtual function still execute C::setup, this is desired for pure virtual method callFunction derived from class A
        a->callFunction("Hello World", 987);
        
        return 0;
        
    }