The short answer is yes.  If you’re in any doubt, put a battery in it.

This article attempts to explain why you should put a battery in your ONT and what issues you need to consider.

With a battery in the ONT, your telephone service should be as reliable as the service you were getting from Telstra (or any other phone company).  The NBN equipment that your ONT is connected to, in the exchange, all runs on UPS equipment and won’t go off line in a power cut.

Black Out

In the event that the power goes out your ONT won’t work unless there’s a battery in it.  If the only service you get on your ONT (NBN) is internet access or pay TV then you might not care. (Your computer and TV won’t work anyway.)

Blackouts generally happen for two reasons, something goes wrong with the power system or someone turns off your power on purpose.

NBNCo say the battery should last 4 hours.  However there has been discussion that suggests the battery should hold the ONT on line for up to 9 hours.  So this should mean you don’t wake up to find the battery and phone died over night.

The ONT will also beep when the power goes out unless you’ve turned the beep function off which we understand you can.

Security Systems

If you have a ‘monitored’ security system that uses the telephone service on your ONT then you need a battery.  Your security system already has a battery in it.

If someone attempts to break in to your home they may well turn off the power first.  This is why your security system has a battery and why your ONT should have one as well.

Emergency Calls – 000

Chances are these days that if you need to make an emergency call you’ll use your mobile.

However, if you’re mobile battery is flat at the time of an emergency (which trust me can happen) then you’ll be reaching for your normal “landline” phone (assuming that you’re not using a cordless phone which has a powered base unit – which will not be working during a power outage unless you also have that plugged in to a UPS).

Medical Alert Systems

If you have one of those ‘pendant’ emergency systems that calls for help at the touch of a button then chances are it uses the telephone service to call for help.  The chances of taking a trip and fall when the power is out, and it’s dark, are higher than when you can see with the lights on, that’s just common sense.  If in doubt, put a battery in the ONT so you know the phone service will be working during a power outage.  (If you are using a Medical Alert System then it would be wise to contact your supplier just to confirm it works correctly, as expected, with your ONT.  It would also be wise to perform a test, they should be able to help you with this.)

Battery Life

Batteries in ONTs, UPSs and security systems don’t last for ever.  They need to be replace every 3 to 5 years depending on the battery.  You can get some that are rated to last longer and the price of the battery is likely to reflect its quality and life.

The ONT should beep when the battery needs changing but this can’t always be relied on.

Battery Monitoring

The ONT should beep a bit like your smoke detector when the battery needs to be replaced, however it is a good plan to do a test from time to time by unplugging the power to the ONT to see if it keeps running while the power is off.

At the time of writing, there is no service that will monitor your ONT battery for you and do a service call automatically when the unit needs changing.  If you feel you need this level of service you should talk to your telephone provider about what they can offer you and choose a provider based on the needs you feel you have.

Update:  NBNCo are talking about providing a monitoring service, though no details have been released as yet – 2/11/10

Where to get batteries

Your telephone provider (RSP) should help you with this.   A number of the Whirlpool posts here may be helpful:  http://whrl.pl/RcvcPG

How much do batteries cost

At the time of writing, batteries should cost around $35 to $50.  You may consider this expensive considering that the battery also needs to be replaced every few years.  There has been some debate about this issue.  Consider this, at the time of writing, if you shop around, the difference between NBN provided telephone services and the ones on the old PSTN network differed by up to $50 per month.  That’s $1800 over three years – a battery costs $50.

How to replace batteries

Instructions on how to replace the battery can be found here.  If you’re not confident with changing the battery in your alarm system or car then you might want to consider getting someone to do it for you.  It’s not complex to do and is reasonably safe but care should still be taken.  Your telephone provider RSP should help you with this.

How to dispose of old batteries

Batteries should not be just thrown in the rubbish, they are very toxic and should be disposed of in the same way you dispose of an old car battery.

UPS Units

If you expect your cordless phone to keep working when the power goes off you also need to have that connected to a UPS unit.  Cordless phones don’t use much power so your average UPS unit will keep one running for some time.

If you are using a Medical Alert System it may also need to be plugged into a UPS.  You should talk to your provider about this.

Summary

If in doubt, put a battery in it.  As with any service change, there are a few things to consider.  You could use this information like a quick check list of things to do.