Wiring a Arduino to a 12v TTY
You'll need to convert the DSC-1555 Alarm systems 12V levels down to something the Arduino can handle.

Without having prior knowledge of how the DSC-1555's serial hardware works, there's not much I can say. I'd get a hold of the datasheet and get more info.

Is it RS-232C (or some other revision) compliant?  If so, you should get a 12V RS-232 -> 5V line converter.  You could try "stepping down" the voltage with resistors to something the Arduino can handle, but this is fraught with problems, as the signals will likely be inverted and may be outside the range the CMOS chip can handle, potentially causing damage.  If it's RS-232 compliant, get a converter and your headaches will be far less.