NumberToWords.cs

using System;
using System.Collections.Generic;
using System.Linq;

namespace NumberToWords
{
	class Program
	{
		private static Dictionary<int, char[]> charDict = new Dictionary<int, char[]>() {
				{2, new char[] { 'a','b','c'}},
				{3, new char[] { 'd','e','f'}},
				{4, new char[] { 'g','h','i'}},
				{5, new char[] { 'j','k','l'}},
				{6, new char[] { 'm','n','o'}},
				{7, new char[] { 'p','q','r', 's'}},
				{8, new char[] { 't','u', 'v'}},
				{9, new char[] { 'w','x','y', 'z'}},
			};

		static void Main(string[] args)
		{
			var wordList = ConvertToWords(23439684987);

			for (int i = 1; i < wordList.Count; i++)
			{
				Console.WriteLine(i + ": " + wordList[i]);
			}
			Console.WriteLine("Total Words: {0}", wordList.Count);
			Console.WriteLine("Distinct Words: {0}", wordList.Distinct().Count());
			Console.ReadKey();
		}

		private static List<string> ConvertToWords(long number)
		{
			List<int> numList = number.ToString().ToCharArray().Select(x => (int)Char.GetNumericValue(x)).ToList();
			var words = new List<string>();
			
			while (numList.Count > 0)
			{
				words.AddRange(Permute(numList, "", 0));
				numList.RemoveAt(numList.Count - 1);
			}

			return words;
		}

		private static List<string> Permute(List<int> nums, string partialWord, int index)
		{
			List<string> words = new List<string>();
			char[] chars = charDict[nums[index]];

			if (index < nums.Count - 1)
			{
				for (int i = 0; i < chars.Length; i++)
				{
					words.AddRange(Permute(nums, partialWord + chars[i], index + 1));
				}
			}
			else
			{
				for (int i = 0; i < chars.Length; i++)
				{
					words.Add(partialWord + chars[i]);
				}
			}

			return words;
		}
	}
}