Hi, I am not very good at programming, which you will be able to tell with how simple this problem is. (This is not an actual homework assignment, just a similar one so I can try to figure it out) But here is the problem.

People sometimes give their telephone numbers using one or more alphabetic characters. Write a program that accepts a 10 digit telephone number that may contain one or more alphabetic characters. Display the corresponding number using numbers. The numbers and letters are associated as follows on your telephone:
ABC: 2
DEF: 3
GHI: 4
JKL: 5
MNO: 6
PQRS: 7
TUV: 8
WXYZ: 9

If the user enters a character that is not on the telephone as a part of the number, display a message indicating that the value does not match a number. Allow both upper and lowercase characters to be entered.

Here is my embarassing work that I have so far. Like I said I am very bad at programming and would love some help! Feel free to start from scratch if you can't build off my work.
Code:
using System;

class DoLoop
{
    public static void Main()
    {
        string myChoice;

        do
        {
            
            Console.WriteLine("Telephone Key\n");

            Console.WriteLine("ABC = 2");
            Console.WriteLine("DEF = 3");
            Console.WriteLine("GHI = 4");
            Console.WriteLine("JKL = 5");
            Console.WriteLine("MNO = 6");
            Console.WriteLine("PQRS = 7");
            Console.WriteLine("TUV = 8");
            Console.WriteLine("WXYZ = 9");

            Console.WriteLine("Please enter a 10 - digit telephone number using the letters above.");

            
            myChoice = Console.ReadLine();

            // Make a decision based on the user's choice
            switch (myChoice)
            {
                case "ABC":
                case "abc":
                    Console.WriteLine("2");
                    break;
                case "DEF":
                case "def":
                    Console.WriteLine("3");
                    break;
                case "GHI":
                case "ghi":
                    Console.WriteLine("4");
                    break;
                case "JKL":
                case "jkl":
                    Console.WriteLine("5");
                    break;
                case "MNO":
                case "mno":
                    Console.WriteLine("6");
                    break;
                case "PQRS":
                case "pqrs":
                    Console.WriteLine("7");
                    break;
                case "TUV":
                case "tuv":
                    Console.WriteLine("8");
                    break;
                case "WXYZ":
                case "wxyz":
                    Console.WriteLine("9");
                    break;
                default:
                    Console.WriteLine("The value you have entered does not correspond with a number.", myChoice);
                    break;
            }

            // Pause to allow the user to see the results
            Console.Write("press Enter key to continue...");
            Console.ReadLine();
            Console.WriteLine();
        } while (myChoice != "Enter" && myChoice != "enter"); // Keep going until the user wants to quit
    }
}