GSMComm message received 321 occurred

Recently I've encountered this error while developing a gsmcomm powered device up couple with 500 error.

To know what these errors mean, you can refer to:
http://www.developershome.com/sms/resultCodes2.asp#16.2.1.1.Table of +CMS Error Codes and Their Meanings|outline

I encountered the error while reading sms messages:

var messages = _comm.ReadMessages(PhoneMessageStatus.ReceivedUnread, PhoneStorageType.Sim);

//later on, I found out that my modem iTegno doesn't support ReceivedUnread memory. So to fix the error I replaced it with PhoneMessageStatus.All.

var messages = _comm.ReadMessages(PhoneMessageStatus.All, PhoneStorageType.Sim);