MyFirstMAUI/MyFirstMauiApp/Phoneword/PhonewordTranslator.cs

using System;
using System.Collections.Generic;
using System.Linq;
using System.Text;
using System.Threading.Tasks;

namespace Phoneword
{
    internal class PhonewordTranslator
    {
        public static string ToNumber(string raw)
        {
            if (string.IsNullOrEmpty(raw)) 
                return null;

            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);
                    else
                        return null;
                }
            }

            return newNumber.ToString();
        }

        private static readonly string[] digits =
        {
            "ABC", "DEF", "GHI", "JKL", "MNO", "PQRS", "TUV", "WXYZ"
        };

        private static int? TranslateToNumber(char c)
        {
            for (int i = 0; i < digits.Length; i++)
            {
                if (digits[i].Contains(c))
                    return i + 2;
            }

            return null;
        }
    }
}