using System;

using System.Text;

using System.Linq;

using System.Collections.Generic;

using System.Text.RegularExpressions;   

​

public class Program

{

    public static void Main()

    {

        string input = "this is your sensitive digit 12345 , if you have any question please call +91999999999999 and my home number is +123 3241 432423";

​

        Regex rgx = new Regex(@"\d{4,}\d");

        string replacedstar = rgx.Replace( input, "*****", 1 );

        //Console.WriteLine(replacedstar);

        Console.WriteLine(MaskDigits(input));

    }

    public static string MaskDigits(string input)

    {

        var split = input.Split("call");

        StringBuilder sb = new StringBuilder();

        if(split.Length > 2)

        {

            Regex rgx = new Regex(@"\d{4,}\d");

            string replacedstar = rgx.Replace( split[0], "*****");  

            return replacedstar + "call" + string.Join("", split.Skip(1).ToList());

        }

        return input;

    }

}