C# で静的メソッドから非静的メソッドを呼び出すときに「オブジェクト参照が必要です...」というメッセージが表示されるのはなぜですか?

namespace ConsoleApplication1
{
    class Program
    {
        static void Main(string[] args)
        {
            Console.Write("Write a number: ");
            long a = Convert.ToInt64(Console.ReadLine()); // a is the number given by the user

            long av = volteado(a); // av is "a" but swapped

            if (siprimo(a) == false && siprimo(av) == false)
                Console.WriteLine("Both original and swapped numbers are prime.");
            else
                Console.WriteLine("One of the numbers isn't prime.");
            Console.ReadLine();
        }

        private static bool siprimo(long a) // Declare siprimo as static
        {
            // Evaluate if the received number is prime
            bool sp = true;
            for (long k = 2; k <= a / 2; k++)
                if (a % k == 0) sp = false;
            return sp;
        }

        private static long volteado(long a) // Declare volteado as static
        {
            // Swap the received number
            long v = 0;
            while (a > 0)
            {
                v = 10 * v + a % 10;
                a /= 10;
            }
            return v;
        }
    }
}