Here's an update. Some more information which I wasn't aware of.
I've been 'setting the alarm off' as a test, to test the system as it would work under an intrusion.
The reason why the communicator output wasn't triggering the dialler is because I've been using my hallway/circulation PIR to get the alarm ringing. This is an entry/exit zone and regulations says that an entry/exit zone cannot trigger a communicator. But it could trigger the communicator if it were a guard circuit.
That seems like a load of rubbish to me, as why does a regulation say that if someone bashes in my front door, runs through the house etc, the siren will go off to tell my neighbours, but I'm
not allowed to know via the dialler calling my phone? Crazy idea.