I'm using my GSM1400 for extensive SMS communications. I service each SMS with the standard "available()" library function which tests for unread messages only. The order in which you service texts is dependent on the service provider and the order in which they send them to your device. I also test for the return value of endSMS() to insure my text was sent properly. I've had issues with the SMS system hanging and it simply never comes back. I use a watchdog timer to keep it from going off into lala land.
// Proper Modem Shutdown MODEM.sendf("AT+CFUN=15"); MODEM.waitForResponse(10000);