Turns out I didn't know quite a lot, school day for me
I've always wondered how those SOS buttons work, I assumed they were voice only and not data.
In that case a bit of further reading from the highways agency:
Manual activation of the in-vehicle emergency call system
If you need to summon help as you are unable to follow our breakdown advice and move to an emergency area:
Keep your seatbelt on.
Turn your hazard warning lights on.
Activate your vehicle’s built-in emergency call system by pushing the eCall SOS button to contact a 999 operator.
The 999 operator will automatically receive your vehicle details including your location and direction of travel. Through your vehicle’s speaker system, they will then ask for further information to establish the nature of the emergency.
You can also use this system to report debris on the road or a vehicle travelling in the wrong direction.
If the emergency call system fails to work when you press the eCall SOS button, use your phone to call 999 for help.
eCall must only be used in an emergency requiring the police, fire service or an ambulance.
Do not use the emergency call system or press the eCall SOS button if you only require breakdown recovery or vehicle roadside assistance.
Automatic activation of the in-vehicle emergency call system
If you are involved in an incident that triggers your airbags:
your vehicle sensors will automatically activate the onboard emergency call system and send your vehicle details including your location and direction of travel when contacting a 999 operator
the 999 operator will use your vehicle’s built-in safety feature to speak to you, and ask for further information to respond quickly and appropriately
If you can, clearly tell the 999 operator you need urgent assistance.
If you are unable to respond to the 999 operator, your vehicle details will be automatically passed to the police.