Summary / TL;DR

999 or 112? It doesn’t matter – Ofcom say “‘999’ and ‘112’ are treated exactly the same way”.
Can I call 999/112 if my home mobile network has no signal? Yes, if another network is available, but emergency services won’t be able to see your number or call you back.
Can I call 999/112 from a locked mobile phone? If the phone’s software supports it, yes (iOS and Android both do).
Can I call 999/112 on a Pay As You Go phone with no credit? Yes, according to Ofcom
Can I text 999/112? Yes, but you need to register first.
Will the 999 operator automatically know where I am? Yes, usually within a relatively large area (mobile phone mast info). If your mobile has iOS 11.3 or Android 9 (Gingerbread) then your phone will try to automatically send more accurate GPS coordinates to emergency services.

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There’s lot of information and mis-information online about dialling 999 or 112 in a UK mountain emergency, which one might (or mightn’t) be better, and how much location information your phone can provide. This post is my description of the situation, which I hope differs from others in that I’ve given trustworthy references to support the statements, instead of just claiming to know things from my head. All of this info was researched from January to May 2018, and is based on the scenario of using a UK-registered mobile phone on the hill in the UK. Please let me know if you’ve any issues with the information provided (with references 😉 ).

999 or 112?

I couldn’t find anything definitive online here, so I asked Ofcom. They replied, by email:

The numbers ‘999’ and ‘112’ are treated exactly the same way – networks favour no number over the other and the call is handled in exactly the same way throughout.

Emergency Roaming

Many people know that if you dial 999 or 112 and your own mobile network has no signal, you can automatically ‘roam’ onto other available networks to make the emergency call. The technical name for this is “Limited State Service” (LSS). Ofcom have confirmed to me that when you do this, the call handler cannot see what your phone number is. Even if they saw the number, they are unable to call you back via any network other than your own home network, which by definition doesn’t have any service if you’re using LSS. You therefore should not expect a call back if you ring 999 or 112 while in LSS; make sure you have given and received all the appropriate information before hanging up!

While you can dial 999 or 112 from a locked mobile phone, or from a Pay As You Go phone which has run out of credit, it’s not possible to connect from a handset that does not contain a SIM card. Ofcom have also confirmed these points by email.

Other references: The Ofcom consultation incorporating LSS was held in March 2009, and the LSS system was activated in October 2009 [para. 3].

Emergency Text Messages

You can text 999 in an emergency, if you have pre-registered beforehand. You can register at www.emergencysms.org.uk, where there are also instructions on how to text 999. Note that texts to 999 are just normal text messages, which means you need signal on your home mobile network – you cannot use LSS to send a text. The system was primarily designed for use by people who cannot use voice/audio systems, rather than people whose phone signal is so poor that they can only text.

A personal note of caution here is that the responses you get by text seem to follow the same script as an operator would use on a phone call, but without the flexibility that human conversation provides. When I had to use the system on Snowdon, I was asked for the address of the emergency. When I replied that there wasn’t one, I was then asked for the postcode. All of this in a blizzard, while keen to return to the casualties I’d left back up the hill.

Location – Pinging

I’ve seen several references online to emergency call handlers being able to press a button and magically ‘ping’ your phone to make it send them location details. I’ve been able to find absolutely zero evidence to support these claims. The call handler will usually receive mobile phone mast info, but that gives a circle  of approx. 2.8km radius (see image below). There’s nothing which lets them actively press a button and silently ‘ping’ your phone to make it send them your exact location. Such a system would depend on any number of factors, including the capabilities of your phone, the nature of your signal availability (voice only, data, etc.) – the options listed below are the next-best thing to magic that we can come up with at the moment though.

Location – AML

Advanced Mobile Location is a relatively new mobile phone function which kicks in silently when you make a 999 call. It is far, far more accurate than other systems, giving emergency services a much smaller circle on the ground in which you are likely to be found (see image below). Your phone will check there’s enough battery remaining, and if so will activate wifi and GPS to work out its location as accurately as possible. It’ll then text the details to the emergency services. It doesn’t need a 3G/4G data connection, and it will turn wifi and GPS off again afterwards so that your battery isn’t drained. The text message sent won’t be visible on the handset. All of this happens without you doing or seeing anything. The call handler does not need to do anything to instigate the process; it happens automatically on your phone. Ofcom told me that “With Advanced Mobile Location (AML), the handset’s best available location information is made available to the person who answers the call.” Note that’s “the person who answers the call” – they then need to pass the information to mountain rescue (see below). AML is available on Google Android from version 2.3 (Gingerbread) onwards, and on Apple iOS from version 11.3 onwards. If you want all the gory details about how it works, check out this separate post I wrote.

Location – SARLOC

SARLOC is a system employed by some mountain rescue teams, which sends an internet link to a casualty via SMS. It’s nothing to do with the initial call to emergency services, but it is frequently used by MR teams so I thought it worthy of inclusion. The internet link sent by MR allows the casualty to open a web page on their phone, which then sends the GPS location of the handset directly to the MRT. Obviously this is reliant on the handset having a sufficient mobile data connection (GPRS/EDGE/3G/4G) to load the web page in the first place, and having the geolocation feature active in the web browser (as described here). It also necessitates a lag between the initial 999 call and establishing the casualty’s position. It has the advantage that it is handset- and OS-agnostic. SARLOC is not available to the public; the initial texts are sent to casualties by MRT as-and-when required.

Location – OS Locate

OS Locate is the Ordnance Survey app for mobile phones, which gives a six-figure grid reference for your current position. It has no link to emergency services, but the reason I’ve included it here is that a lot of people don’t realise that you don’t need phone signal for it to work (see question 6 on the official FAQ). So even if you don’t have any service on your home mobile network, you could still use it to confirm your position before trying to make an emergency call using LSS, or moving to a location where you do get signal.

Location – what3words

what3words is a service which divides the planet up into 3 metre squares and gives each one a unique combination of three English words. It has the advantage of being very precise, and their app makes it very easy to find and share your current what3words address. A collection of FAQs very neatly outlines the clever model underlying the system, which is more complex than you’d think. One potential disadvantage of the system is that it’s entirely proprietary, meaning that if what3words suddenly decide to start charging for the system, or go bankrupt and cease to operate, then access would just terminate. There has also recently been at least one case of an incorrect location being given to Mountain Rescue by casualties using What3Words.

Location – Passing to MR

All of the above scenarios relate to your mobile phone sending location information to the emergency services call handler. I have had a couple of conversations with people from different mountain rescue teams, both of whom have said that there is no standard procedure for the police to pass an agreed dataset – including, for example, location information – to MR when notifying them of a call. I will admit I’m starting to stray into anecdotal evidence rather than concrete references here, but my advice would be to make it clear to the call handler that the grid reference you give them, for example, is a critical piece of information that must be given to MR.

The Caveat

Technology is great, and I love things like AML maximising the potential of the handsets almost all of us carry around with us. However, things go wrong and batteries do run out. The first time I had to dial 999 on the hill, the speaker stopped working as wet snow got into it, so the call handler could hear me but I couldn’t hear them. Don’t just assume that the call handler is definitely going to automatically receive your location info – always back things up with a grid reference as part of the call. It’s obvious advice, but it’s up there with other things that shouldn’t necessarily need saying like “take a map and compass” and “know how to use them”.