/// <summary>
    /// 替换手机号中间四位为*
    /// </summary>
    /// <param name="phoneNo"></param>
    /// <returns></returns>
    protected string ReturnPhoneNO(string phoneNo)
    {              
        Regex re = new Regex("(\\d{3})(\\d{4})(\\d{4})", RegexOptions.None);
        phoneNo = re.Replace(phoneNo, "$1****$3");
        return phoneNo;
    }

------解决方案--------------------
你的item.phone应该是字符串吧

DH = item.phone;
string str = item.phone;   //将item.phone转换成字符串
str = str.subString(3,4);          //截取第4个数后的4个数。
DH = DH.replace(str,"XXXX");