public GSM(string model, string manufacturer, decimal price,
     string owner, Battery baterry, Display display)
 {
     this.Model = model;
     this.Manufacturer = manufacturer;
     this.Price = price;
     this.Owner = owner;
     this.BatteryCharacterisitcs = baterry;
     this.DisplayCharacterisitcs = display;
     this.CallHistory = new List<Call>();
 }
 public GSM(string model, string manufacturer, decimal price, Display display)
     : this(model, manufacturer, price, null, null, display)
 {
 }