Did you know that you can text 999? If mobile signal won't support a voice call but is strong enough to send/receive texts, you can still alert the emergency services. Remember that in the Highlands you don't need to be remote to be in a poor signal area.
You will need to register your number with the service so do it now. Simply text the word register to 999 and follow the instructions in the reply.
In an emergency, include in your text: - Your location, ideally a grid reference (remember the OS Locate app?) - The number and ages of casualties - Details of injuries - Current weather and ground conditions - Details of the equipment the group has: group shelter, food, torches, clothing etc - Your mobile number and any other mobile numbers in the group