public void Functionality()
{
Gsm.AddCall(new Call(new DateTime(2016, 06, 09, 12, 00, 05), "0888 88 88 88", 100));
Gsm.AddCall(new Call(new DateTime(2016, 06, 06, 13, 04, 10), "0777 77 77 77", 200));
Console.WriteLine(Gsm.CallHistory);
Console.WriteLine("Total price: {0} BGN", Gsm.GetPrice(PricePerMinute));
Gsm.RemoveLongestCall();
Console.WriteLine(
"Total price after removing longest call is {0} BGN", Gsm.GetPrice(PricePerMinute));
Gsm.ClearHistory();
Console.WriteLine(Gsm.CallHistory);
}