I'm using sms3 daemon with a pool of modems, and the messages need to be queued in a certain way depending on some circumstances. When a modem is suddenly not usable (switched off, SIM-card taken out or internal error in the modem, no matter) I need to know of the fact to no more send messages to it.
What are most robust and simple ways to know that?

I suppose that I might use alarmhandler and detect all the errors about modems (using this error list: http://smstools3.kekekasvi.com/index.php?p=error-messages), but maybe there is a simpler way yet still robust enough?

Thanks in advance to anyone who lets me know.