sixwheeledbeast

Removing the hold-off for a minute or so is enough to check the battery. If the sound dips or has a low output it's likely it's near to failure. They are Nicd so last longer than the lead acid type batteries in the panel. Depending on the box you can't disable the sounder triggering. It will have at least one fixing screw you can't have something like that in the elements only clip on.

Yes, if you know how to wire them correctly and you don't overload the panel, as I explained in post 2

Two live boxes could have been too much current and blown something. Most panels need additional PSU's or SCB mode for multiple sounders.

An additional power supply is not required and you do need the correct expander card. I would still use Low Current and/or SCB mode on the bell myself to reduce the current drawn by the sounder. The expander card comes with instructions, it requires an EOL resistor in the sounder but other than that it's simple. Yes, the GSM sends the same type of signals as PSTN, text message have zone information which is good. You have to set them up correctly with test signal enabled otherwise the SIM will expire. This is a one-way systems so not the most secure, would have to be a low risk site IMO.

IIRC on an ELite 24 the default programming for Pin 3 is Intruder Alarm.

Digicom Output 3 if you don't fancy doing any programming.