A phone number is not an evacuation plan.
A plan agreed in advance fixes the departure airports, the aircraft types, the muster points and a single number to call, before anything has happened. It is cheaper to plan than to react, and considerably calmer.
Organisations with staff in difficult places generally carry a duty of care obligation. A documented plan satisfies it in a way an emergency number does not, and it is the difference between a decision and a scramble.
