public void DeleteEntry(Call entry)
 {
     callHistory.Remove(entry);
 }
        static void Main(string[] args)
        {
            GSM newGSM = new GSM("Samsung", "Note III", 1400, "Me", new Battery("ChinaMax", 240, 120), new Display(5, "123j", 256));

            Call firstCall = new Call("0888888888", 124);
            Call secondCall = new Call("0888888888", 13);
            Call thirdCall = new Call("0888888888", 56);

            newGSM.AddEntry(firstCall);
            newGSM.AddEntry(secondCall);
            newGSM.AddEntry(thirdCall);

            foreach (Call call in newGSM.CallHistory)
            {
                StringBuilder sb = new StringBuilder();
                sb.Append("The date of the call is : " + call.DateOfCall + "\nThe time of the call is : " + call.TimeOfCall + "\nContacted phone number : " + call.DailedPhoneNumber + "\nThe duration of the call is : " + call.DurationOfCall + " minutes\n");
                Console.WriteLine(sb);
            }

            newGSM.PhoneBillForAllCalls(0.37M);

            newGSM.DeleteEntry(firstCall);

            newGSM.PhoneBillForAllCalls(0.37M);

            newGSM.ClearCallHistory();

            foreach (Call call in newGSM.CallHistory)
            {
                StringBuilder sb = new StringBuilder();
                sb.Append("The date of the call is : " + call.DateOfCall + "\nThe time of the call is : " + call.TimeOfCall + "\nContacted phone number : " + call.DailedPhoneNumber + "\nThe duration of the call is : " + call.DurationOfCall + " minutes\n");
                Console.WriteLine(sb);
            }
        }
 public void AddEntry(Call entry)
 {
     callHistory.Add(entry);
 }