public void CallTest()
        {
            //Creating GSM instances
            List<GSM> samsung = new List<GSM>();
            samsung.Add(new GSM("Galaxy S2", "Samsung"));
            samsung.Add(new GSM("Galaxy S3", "Samsung"));
            samsung.Add(new GSM("Galaxy S4", "Samsung"));
            GSM nokia = new GSM("Lumia 520", "Nokia");

            //Adding calls
            List<Call> callsHistory = new List<Call>();
            callsHistory.Add(new Call("0883339897", 78));
            callsHistory.Add(new Call("0883352629", 134));
            callsHistory.Add(new Call("0883339897", 61));

            nokia.AddCall(callsHistory[0]);
            nokia.AddCall(callsHistory[1]);

            // Display the info about the calls
            Console.WriteLine(nokia.ToString());
            Console.WriteLine(callsHistory[0]);
            Console.WriteLine(callsHistory[1]);
            Console.WriteLine();
            foreach (var item in callsHistory)
            {
                Console.WriteLine(item);
            }
            Console.WriteLine(nokia.TotalPrice(callsHistory));

            Console.WriteLine();
            Console.WriteLine();
            // Total price in call history

            RemoveLongestCall(callsHistory);

            foreach (var item in callsHistory)
            {
                Console.WriteLine(item);
            }
            Console.WriteLine(nokia.TotalPrice(callsHistory));
        }
        public void Test()
        {
            //
            //View array instances
            //
            GSM[] gsmArray = new GSM[4];

            gsmArray[0] = new GSM("3310", "Nokia", "Pesho", 15, new Display("1,25 inch", 2), new Battery("Unknown"));
            gsmArray[1] = new GSM("Moto G", "Motorola", "Matrin Kolev", 320);
            gsmArray[2] = new GSM("Galaxy S5", "Samsung", "Eli", 700, new Display("5 inch"), new Battery("High capacity", 700));
            gsmArray[3] = new GSM("Z5", "Sony", "Plamen", 600);

            //
            //Printing the GSM Class instances
            //
            foreach (var item in gsmArray)
            {
                Console.WriteLine(item);
            }

            Console.WriteLine(GSM.iphone4S);
        }