using System;

public class Program

{

    public static void Main()

    {

        var phoneNumber1 = "01234567899";

        var phoneNumber2 = "+441234567899";

        var phoneNumber3 = "+441334567899";

​

        Console.WriteLine("The numbers {0}match",IsMatch(phoneNumber1,phoneNumber2)?"":"do not ");

        Console.WriteLine("The numbers {0}match",IsMatch(phoneNumber1,phoneNumber3)?"":"do not ");

    }

    //*

    public static bool IsMatch(ReadOnlySpan<char> a, ReadOnlySpan<char> b)

    {

        var a10 = a[^10..];

        var b10 = b[^10..];

        return a10.Equals(b10, StringComparison.Ordinal);

    }

    //*/

    // Does NOT work!

    /*

    public static bool IsMatch(ReadOnlySpan<char> a, ReadOnlySpan<char> b)

    {

        return a[^10..] == b[^10..];

    }

    //*/

}