We've got six Aico interconnected smoke alarms (from the Ei140RC series) and feel well protected in that respect inside the house. What would give me extra peace of mind is receiving notification (e.g. SMS) when I am out of the house should they ever go off and was wondering if anyone here had tapped in to the interconnect line and knew what form it took?

They are mains powered but have battery backup and so my starting assumption is that it is probably a 9v DC signal on that line when alarmed?