It doesn’t always require a new connection method. If you have FTTP you can purchase a second “VOIP” line and have the alarm system plugged into that port. The reason for registering your equipment was/is so that NBN Co can advise you about whether the system will support your equipment, if using FTTP the fitting of the backup battery, and advise you of outages that may affect you. I have Medical Alarm Equipment and have not had to register it with NBN as my RSP does not support Priority Assistance (regardless my RSP fits the battery as a standard if FTTP is used).
If you have a pendant alarm type system that uses your existing phone to dial out you can still use it with FTTN, HFC, Fixed Wifi, FTTP, FTTB, FTTC. It will however possibly be less reliable than a current PSTN phone (power outage issues). A lot of this power reliability can be somewhat averted by the use of in home power backup to the power supply sensitive equipment, for example plugging the affected unit into a UPS, using a phone that will work off it’s own power supply (some linked to in another thread) in a blackout.
Many businesses that supply equipment are changing to Mobile connected as 1. it can just be cheaper, 2. not everyone can use a second voip line, or 3. they just don’t understand the connected premises (they don’t have to worry about site inspections).
Mobile connectivity is not the magic bullet either. There are many situations where to use mobile is very difficult if not impossible. It still requires power backup, mobile coverage (which may also necessitate external antennas), space requirements, retrofitting compatibility.
I think if alternative connections (other than HFC) are used instead of FTTP, I think the customer/consumer should at least be advised the modems should be power backed up and offered advice about which phones could be used in a blackout (power outage).
Of course HFC users face a worse dilemma than many other NBN users as most of the HFC infrastructure close the the houses are not power outage resilient at all.
FTTN & FTTB nodes are now mostly standby power backed up and FTTC can easily be power backed up from the home with the DPU in the pit being already powered by the RPF (Reverse Power Feed Unit) that is installed in the house and that unit can be connected to a standby power supply (the DPU is a low power consumption unit). Wifi Towers are supposed to be Standby Power backed up and so only the home infrastructure needs to be connected to standby power. Satellite only requires the home installed equipment to be Standby Power backed up to continue to maintain communications (weather permitting).
This is something that as the NBN/RSP contracts to connect your home should be advising you of and helping you get recommendations and or giving you recommendations to alleviate any issues.
Your connected equipment will determine the size of the backup power unit/s you require and if you are not sure it is best to get the advice of a professional who is familiar with power usage requirements.