today I use a Gsm modem to notify zabbix alerts by sms , the organization that I work there have billionton GSM Modem:
for using this device , we must do this:
First find my usb that used by dmesg command. my used usb is /dev/ttyUSB0.
Edit sms media type :Administratopn->Media Type
change media for user in Administrations->Users
Finally add a action like this Post: Script media
NOW , if you have an error like this : error in open(/dev/ttyUSB0)  permission denied
you must do this is terminal : chmod 777 /dev/ttyUSB0
Enjoy your life!!