Text messages require less signal strength than voice so they can get through even when the connection is flaky or apparently non existent. If you have an accident out walking, you may not have enough signal to call 112 or 999 but if your phone is registered with the emergency sms service you an send a call for help to 112 or 999. Message should state: service required, what has happened and location. The operator will pass the message to the emergency services and text back a reply.
Use the link provided to register your mobile phone with the service. Better to do it now than wait until you are out in the wilds and needing help.