using System;

using System.Text.RegularExpressions;

public class Program

{

    private static readonly Regex invalidPhoneNumberRegex = new Regex(@"[\+\(\)\-\. ]", RegexOptions.Compiled);

    private static readonly MatchEvaluator matchEvaluator = match => { return string.Empty; };

    public static void Main()

    {

        const string PhoneNumber = "+1 (425).999-6318";

        Console.WriteLine(PhoneNumber);

​

        var stripped = invalidPhoneNumberRegex.Replace(PhoneNumber, matchEvaluator);

        Console.WriteLine(stripped);

    }

}