static void Main()
        {
            Random randomGenerator = new Random();

            GSM NokiaMobile = new GSM("Lamia", "Nokia");
            GSM SamsungMobile = new GSM("Grifin", "Samsung");
            GSM HtcMobile = new GSM("Eno", "HTC");

            for (int i = 0; i < 10; i++)
            {
                DateTime callDate = DateTime.Now.AddSeconds(randomGenerator.Next(1000000));
                string mobileNumber = randomGenerator.Next(800000000, 1000000000).ToString();
                int durationInSeconds = randomGenerator.Next(3600);
                Calls someCall = new Calls(callDate, mobileNumber, durationInSeconds);
                NokiaMobile.AddCall(someCall);
            }
            NokiaMobile.DisplayCalls();

            RemoveLongestCall(NokiaMobile);

            Console.WriteLine("\n ==== Longest call removed ==== \n");

            NokiaMobile.DisplayCalls();

            NokiaMobile.DeleteCallHistory();

            Console.WriteLine("\n ==== Call History Deleted ==== \n");

            NokiaMobile.DisplayCalls();
        }
        private static void RemoveLongestCall(GSM phones)
        {
            long longestDuration = 0;
             Calls longestCall = null;

             foreach (Calls call in phones.PerformedCalls)
             {
                 if (call.Duratoin > longestDuration)
                 {
                     longestDuration = call.Duratoin;
                     longestCall = call;
                 }
             }

             phones.DeleteCall(longestCall);
        }