using static System.Console;

public class Program {

    public static void Main() {

        var texto = "(99) 9999-9999";

        WriteLine(texto.Replace(new string[] {"(", ")", "-", " " }, ""));

    }

}

​

public static class StringExt {

    public static string Replace(this string str, string[] separators, string newValue) {

        foreach (var chr in separators) str = str.Replace(chr, newValue);

        return str;

    }

}

​