gistfile1.cs

    internal class Program
    {
        private static void Main(string[] args)
        {
            IList<CellPhone> o = new CellPhones();
            o.Add(new CellPhone());

            Console.WriteLine("item count:{0}", o.Count.ToString());
        }
    }

    [Serializable]
    public class CellPhone
    {
        public string CountryCode { get; set; }

        public string Phone { get; set; }

        private string _FullCellPhone;

        public string FullCellPhone
        {
            get
            {
                _FullCellPhone = CountryCode + Phone;
                return _FullCellPhone;
            }
        }
    }

    [Serializable]
    public class CellPhones : List<CellPhone>, IList<CellPhone>
    {
        public new void Add(CellPhone item)
        {
            Console.WriteLine("test");
            base.Add(item);
        }
    }