How to wire a GSM module to my alarm system
You'll have to figure out the voltage range on whichever output you want to trigger off of, then construct a suitable buffer between it and the microcontroller comparator (Arduino, in this case). The buffer protects the microcontroller and other digital circuitry from the alarm control panel and vice versa. You wouldn't want a microcontroller programming bug to trigger an alarm or burn out one of the alarm control panel outputs, nor would you want the alarm panel to fry your circuit by relay flyback or a power-cycling artifact.

If the signal voltage is higher than the comparator's maximum input voltage it will have to be divided down by the buffer -- a resistive divider and voltage follower will do, so long as the alarm control panel output can safely source current for the divider. If it's lower (ie: negative), the buffer will have to invert it to the proper range. If it's just right, you can use a voltage follower.

Some options for a buffer:


op amp circuits -- high impedance; can shift voltages; feedback path needs to be designed to not transmit spikes
transistor logic -- a clever circuit can be very simple and effective; not as good isolation as op amps
optical isolation -- excellent isolation; can be done with a pair of LEDs; required alarm control panel circuitry needs failure mode analysis (what if LED shorts? what if signal spikes?)
direct wired connection -- I'm not going there


I'd use an op amp buffer followed by a chip fuse, and clamp the maximum output voltage (tiny MOV). This way even a sustained over-voltage event won't kill or disable the digital circuitry. There are no doubt buffer ICs with all of this. A SMS text saying "the alarm system just went berserk, so it was 'let go'" could then be sent to your phone ;)