private bool IsValidNumber(string str)
   {
       const int VALID_LENGTH = 10;
       bool valid = true;

       if (str.Length == VALID_LENGTH)
       {
           foreach (char ch in str)
           {
               if (char.IsLower(ch))
               {
                   valid = false;
               }
           }
       }
       else
       {
           valid = false;
       }

       return valid;
   }