public void DeleteCall(Call call)
 {
     this.callList.Remove(call);
 }
        //10. Add methods in the GSM class for adding and deleting calls from the calls history.
        //Add a method to clear the call history.
        public void AddCall(Call call)
        {
            if (callList == null)
            {
                callList = new List<Call>();
            }

            this.callList.Add(call);
        }