This sketch can be used as is with the commands I've implemented or modified according to your needs, for example adding more outputs, more features or whatever. It would be fine to share your modifications and/or corrections here to make this project greater.

Let's take a look at the functions implemented so far.
The software considers two kind of users: Superusers and standard one .
The Superusers are granted by writing their phone numbers into the SIM phone book at the first positions. They can also perform the administration commands: check status, manage the phone book etcetera.
The standard users have to be granted by writing their phone number into the phone book, stored permanently in the Arduino EEPROM. They can perform just the standard commands, listed below.

User commands

"ON 1" : OUT 1 = ON (PIN 12)
"OFF 1" : OUT 1 = OFF
"ON 2" : OUT 2 = ON (PIN 13)
"OFF 2" : OUT 2 = OFF
"STATUS" : Request current status of the outputs without any change

The commands are performed sending an SMS with the syntax exactly as written in bold in the list below within quotes (case insensitive).

The sketch uses a software serial library to connect the shield, so the hardware serial can be used for debug connecting the USB cable to Arduino and starting serial monitor.

If enabled by the superuser the execution of any action is confirmed by a phone callback to the calling number.

Both users and Superusers can activate the temporary output (PIN 11) with a simple phone call. If the calling number is recognized in the SIM or Arduino phone book, the system hangs up the line without answering to avoid expenses and activates the I/O for 2 seconds. This method is often used to grant access, temporary or permanently, to guests of any kind of accommodation, to avoid the need of a doorman or the distribution of keys.

Superuser commands (manage phone book)

"PHONE WRITE-xx:nnnnnnnnnn" Write number to grant nnnnnnnnnn into the phone book address xx. Sends back the value read on EEPROM after writing, for verification

"PHONE PAGE-x" Request reading page x (0 to 7) from phone book, 8 numbers per page

"PHONE READ-yy" Request reading granted phone number from address yy (0 to 63) of phone book

"PHONE FORMAT-1234" Phone book format. Fills up all the EEPROM bytes with 0. Sends back a message for confirmation

Superuser commands (other)

"INFO" Request some info from the SIM900 module (operator, signal strength, IMEI)
"ACK ON" Enable Acknowledge call back (phone back to confirm)
"ACK OFF" Disable Acknowledge call back (default)
"PRI ON" Enable privacy (default) (don't show the whole calling number on debug window)
"PRI OFF" Disable privacy