using static System.Console;

using System;

WriteLine(FormataTelefone("49988070405"));

WriteLine(FormataTelefone("4988070405"));

​

static string FormataTelefone(string number) {

    if (number.Length != 10 && number.Length != 11) throw new ArgumentException($"Tamanho da string deve ser 10 ou 11", nameof(number));

    ReadOnlySpan<char> span = stackalloc char[number.Length];

    span = number.AsSpan();

    var offset = 11 - number.Length;

    return $"({span[0..2]}) {span[2..(7 - offset)]}-{span[(7 - offset)..]}";

}