String subStr=phone.Substring(3,4);
oldcellphone.Text = phone.Replace(subStr,"****");

 

 

/// <summary>
  /// 替換手機號中間四位為*
  /// </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;
  }