public string TestCallHistory()
        {
            Call call1 = new Call(Date, StartTime1, Duration1);
            Call call2 = new Call(Date, StartTime2, Duration2);
            Call call3 = new Call(Date, StartTime3, Duration3);
            GSM  gsm   = new GSM(Model, Manufacturer, Price, Owner, BatteryModel, HoursIdle, HoursTalk, BatteryType1, DisplaySze, DisplayColors);

            gsm.CallHistory.Add(call1);
            gsm.CallHistory.Add(call2);
            gsm.CallHistory.Add(call3);

            StringBuilder callHistoryBuilder = new StringBuilder();

            callHistoryBuilder.AppendLine(gsm.GetCallHistory());
            callHistoryBuilder.AppendLine(gsm.CalculateBill(PricePerMinute));
            Call longestCall = gsm.CallHistory.OrderByDescending(c => c.Duration).FirstOrDefault();

            if (longestCall != null)
            {
                gsm.CallHistory.Remove(longestCall);
            }

            callHistoryBuilder.AppendLine(gsm.CalculateBill(PricePerMinute));
            gsm.ClearCallHistory();
            callHistoryBuilder.AppendLine(gsm.CalculateBill(PricePerMinute));
            callHistoryBuilder.AppendLine(gsm.GetCallHistory());

            return(callHistoryBuilder.ToString().TrimEnd());
        }
        public string TestGSM()
        {
            GSM[] gsms = new GSM[]
            {
                new GSM(Model, Manufacturer, Price, Owner),
                new GSM(Model, Manufacturer, Price, Owner, BatteryModel, HoursIdle, HoursTalk, BatteryType1, DisplaySze, DisplayColors),
                new GSM(Model, Manufacturer, Price, Owner, BatteryModel, HoursIdle, HoursTalk, DisplaySze, DisplayColors)
            };

            StringBuilder gsmBuilder = new StringBuilder();

            foreach (GSM gsm in gsms)
            {
                gsmBuilder.AppendLine(gsm.ToString());
            }

            gsmBuilder.Append(GSM.GetNokiaN95());

            return(gsmBuilder.ToString());
        }