namespace PhoneListTest
{
    internal class Program
    {
        static async Task Main(string[] args)
        {
            using (var fs = File.Create("./phonenumbers.txt"))
            using (var sw = new StreamWriter(fs))
            //using (var zipArchive = new ZipArchive(ms))
            {
                foreach (var phoneNumber in GeneratePhoneList())
                {
                    await sw.WriteAsync(phoneNumber);
                }

                await sw.FlushAsync();
            }
        }

        private static IEnumerable<string> GeneratePhoneList()
        {
            for (int i = 0; i < 1_000_000_000; i++)
            {
                yield return i.ToString().PadLeft(15, '0');
            }
        }
    }
}