No alarm signal if connected to a PABX analogue extension.

If the alarm's test buttons work OK but the alarm only gives a very short tone-signal at the start of every ring cycle, the likely cause is that the alarm is unable to detect the ring cadence of that particular PABX. Changing the system's ring cadence may fix the problem. Before reprogramming the system, test the alarm by connecting it across a PSTN line to verify that the alarm's input circuitry is operating correctly.

If the alarm's test buttons work OK but the alarm give only a single short tone-signal when the first burst of incoming ring is received (and is silent thereafter), there's a fair chance that the alarm's input circuitry has been damaged and the alarm unit will need to be replaced.