We don't know if this is a hardware bug or a case of we need to buy a new GSM modem due to it's disfunction, but sometimes our MC35i-based GSM modem gets lost the SIM card inserted, thus failing to send any SMS-alert with

 30932:20110813:205219.038 Write to GSM modem ["
]
 30932:20110813:205219.038 In read_gsm() [> ] [NULL] [NULL] [NULL]
 30932:20110813:205219.246 Read from GSM modem [
+CME ERROR: SIM not inserted
]

We make it work again with such a sequence:

AT+CFUN?
+CFUN: 1

This outputs current functionality level (level 1 [FULL FUNCTION] in this case)

Then:
AT+CFUN=<fun>,<reset> where <fun> is functionality level to set (CFUN? output in my case, mean 1) and <reset> is optional "Reset hardware before setting level", 0 - no reset, 1 - make a reset. So our hardware gets fully functional after

AT+CFUN=1,1

I check my hardware state with:

AT^SCID
^SCID: 89701010050301******

Last line means it has got SIM information correctly (or it'd output 'SIM not inserted' error).

Could this be implemented into zabbix server?

like this:

send_sms(handle, params)
(state, output) = get_send_state(handle)
if not state:
  if 'SIM not inserted' in output:
    reset_modem(handle)
    send_sms(handle, params)

Additionally, if hardware reset didn't help, this could be displayed in web-interface like any other monitored hardware failure (not just only as "Not sent" state of alerts in Action field of a failed node).