using System;
using System.Text.RegularExpressions;
using System.IO;
public class Test
{
	public static void Main()
	{
		var data = "I have this (123) 556-7890 phone number, followed with 123 556 7890, and (123) 556- 7890.";
		var res = Regex.Replace(data, @"\(?\d{3}\)?-? *\d{3}-? *-?\d{4}", 
					x => Regex.Replace(x.Value, @"\d(?!\d{0,3}$)", "X"));
		Console.WriteLine(res);
	}
}