How can I print out all possible letter combinations a given phone number can represent?

Question

I just tried for my first programming interview and one of the questions was to write a program that given a 7 digit telephone number, could print all possible combinations

Answer 1

    private List strs = new List();        
    char[] numbersArray;
    private int End = 0;
    private int numberOfStrings;

    private void PrintLetters(string numbers)
    {
        this.End = numbers.Length;
        this.numbersArray = numbers.ToCharArray();
        this.PrintAllCombinations(this.GetCharacters(this.numbersArray[0]), string.Empty, 0);
    }

    private void PrintAllCombinations(char[] letters, string output, int depth)
    {
        depth++;
        for (int i = 0; i < letters.Length; i++)
        {
            if (depth != this.End)
            {
                output += letters[i];
                this.PrintAllCombinations(this.GetCharacters(Convert.ToChar(this.numbersArray[depth])), output, depth);
                output = output.Substring(0, output.Length - 1);
            }
            else
            {
                Console.WriteLine(output + letters[i] + (++numberOfStrings));
            }
        }
    }

    private char[] GetCharacters(char x)
    {
        char[] arr;
        switch (x)
        {
            case '0': arr = new char[1] { '.' };
                return arr;
            case '1': arr = new char[1] { '.' };
                return arr;
            case '2': arr = new char[3] { 'a', 'b', 'c' };
                return arr;
            case '3': arr = new char[3] { 'd', 'e', 'f' };
                return arr;
            case '4': arr = new char[3] { 'g', 'h', 'i' };
                return arr;
            case '5': arr = new char[3] { 'j', 'k', 'l' };
                return arr;
            case '6': arr = new char[3] { 'm', 'n', 'o' };
                return arr;
            case '7': arr = new char[4] { 'p', 'q', 'r', 's' };
                return arr;
            case '8': arr = new char[3] { 't', 'u', 'v' };
                return arr;
            case '9': arr = new char[4] { 'w', 'x', 'y', 'z' };
                return arr;
            default: return null;
        }
    }