I've just bought a satellite locator as I spend a lot of time in remote locations on my own, out of mobile signal. As said, even if your network can't get a signal your phone will route a 999 or 112 call through other networks if they have coverage but there are still plenty of places in the UK with no signal on any network.
My device, a Delorme Inreach works on the iridium satellite network which is essentially anywhere on earth, as long as you can see the sky. It is waterproof, dust proof and shock proof and if triggered sends my GPS position to an emergency coordination centre in Texas who alert the required local emergency services. It can work as a standalone unit or pair with an iPhone to send and receive text messages, tracking messages etc.
I hope I never need it but if ever I do it will be the finest £200 I ever spent.
Sad news about the photographer, it could happen to any of us, I had a moment in Glen Etive before Christmas which made me think more about how I would cope in an emergency. Even something minor like a broken ankle would become a major issue if you couldn't get out or raise an alarm.