using System;

public static class Program

{

    public static void Main()

    {

        var phoneString = "49988070405";

        // Primeiro exemplo da resposta.

        Console.WriteLine(long.Parse(phoneString).ToString(@"(00) 00000-0000"));

        Console.WriteLine(phoneString.FormatPhoneNumber());

        var phone = 49988070405;

        Console.WriteLine(phone.FormatPhoneNumber());

    }

    public static string FormatPhoneNumber(this long number) {

        return number.ToString(@"(00) 00000-0000");

    }

    public static string FormatPhoneNumber(this string number) {

        return long.Parse(number).FormatPhoneNumber();

    }

}

​

// Resposta no SOpt: https://pt.stackoverflow.com/a/283658/102836