If your phone emits a loud squark and displays an emergency message from Civil Defence between 6pm and 7pm, don't panic - it's just a drill.

Specifically, it's the latest test of the Emergency Mobile Alert system, which is is designed to send a text message to every possible handset in the country - or a targetted area - in the event of a disaster.

This evening's effort will be targetted at every cellphone user on every network, everywhere in the country.

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The first attempt, in October 2017, went haywire as Vodafone customers were sent the txt between 1.30am and 2.30am by mistake - multiple times. The blunder was pegged on the Dutch provider of the technology, one2many.

Why didn't I get it?

Civil Defence subsequently took direct control of the Emergency Mobile Alert system, and a test in November las year was successfully delivered to around 60 per cent of phones.

If you didn't get the message, the most likely reason was that your phone was too old to be compatible.

In terms of Apple, for example, every iPhone from the 5s (released in 2013) forward should receive the message fine. (An interesting sidenote: the iPhone 6 - various sub-models of which were first launched across 2014 and 2015 - is still the single most popular phone on our largest mobile network, Vodafone NZ.)

A complete list of compatible phones is here.

Civil Defence says it expects more than 60 per cent will get the new test. How many? Unknown. That's one of the key reasons for the test.

Why did I get it more than once?

Two emergency alerts were also sent during the recent convention centre fire in Auckland, one on October 22 and the other on October 23.

Some people said they did not get the second alert. Civil Defence says that was probably because they had strayed out of the tightly targetted area (which in rough terms was bordered by Spark Arena in the East, Victoria Park in the West, the Viaduct Basin in the North and spaghetti junction in the south).

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And some people got the alert more than once. Civil Defence said, "If your phone moved from 3G to 4G during this time, you will have received an alert from both networks. The same thing would have happened if you turned flight mode on and off, or turned your phone off and back on during the test broadcast period."

Where the heck did it disappear to?

The convention centre fire alert included links to websites for more information, but a number of people didn't know where to find the message after flashed onto their home screen then disappeared.

On Android phones, the alert may be found in the Messages app.

For iPhone users, the alert will be in your notifications. Access your notifications by swiping down from the top of your screen. If you delete your notifications, the alert will also be deleted.

Why is called a 'Presidential Alert'

This is the name for an emergency text alert system used by US agency FEMA - which pioneered the concept of pushing a message to every cellphone. Civil Defence says not every phone manufacturer has supported a tweak to rename it for NZ. So far. Anyway, don't freak out and think Donald Trump is sending you a txt about a tsunami.

That squark is pretty annoying. Can I opt-out?

Nope

In an emergency, should you phone 111 or 112?

Civil Defence says it never anticipated its Emergency Mobile Alert would reach anything like 100 per cent of the population from the get-go. It's designed to complement other communication vehicles, including news websites, radio and social media.

Speaking of other methods of communication: After a recent debate in the geek community over whether it's best to call 111 or 112 from your cellphone in any emergency, Police posted a notice encouraging people to dial 111.

112 is an international emergency number that works in almost every country, including NZ (where a 112 will be automatically rerouted to 111).

It's not correct that 112 will give you an inside edge getting through. When you dial 111 on your mobile, your phone will also try to reach any available network (that is, if you're a Vodafone customer, it will also try to reach a Spark or 2degrees tower if necessary).

Still, 112 is handy to remember when your travelling, since it works everywhere, and emergency numbers differ (the US is 911 and Australia 000, for example).

In an emergency, it can be better to send a txt than attempt a voice call. A txt is easier for any mobile network to handle, and if there's overloading, Vodafone, Spark and 2degrees' systems will all automatically keep trying to deliver a txt until things clear up.

One final point: in Australia, there's an additional emergency number, 106, which the hearing or speech-impaired can text in an emergency.

Here, you can text 111, but only if you've registered your number, which brings to mind the nightmare scenario of trying to register as an emergency is happening. Surely we can do better than that?