PhoneTranslator.cs

using System.Text;

namespace Core
{
    public static class PhonewordTranslator
    {
        public static string ToNumber(string raw)
        {
            if (string.IsNullOrWhiteSpace(raw)) return "";
            raw = raw.ToUpperInvariant();

            var newNumber = new StringBuilder();
            foreach (var c in raw)
            {
                if (" -0123456789".Contains(c)) newNumber.Append(c);
                else
                {
                    var result = TranslateToNumber(c);
                    if (result != null) newNumber.Append(result);
                }
            }
            return newNumber.ToString();
        }

        private static bool Contains(this string keyString, char c)
        {
            return keyString.IndexOf(c) >= 0;
        }

        private static int? TranslateToNumber(char c)
        {
            if ("ABC".Contains(c)) return 2;
            if ("DEF".Contains(c)) return 3;
            if ("GHI".Contains(c)) return 4;
            if ("JKL".Contains(c)) return 5;
            if ("MNO".Contains(c)) return 6;
            if ("PQRS".Contains(c)) return 7;
            if ("TUV".Contains(c)) return 8;
            if ("WXYZ".Contains(c)) return 9;
            return null;
        }
    }
}