A disaster plan can fail because it assumes everyone begins at home with a charged phone.

Most emergency plans quietly assume a normal starting position: at home, with a charged phone, mobile, informed, and with people nearby. Real emergencies scatter people, drain batteries, and cut communication, and a plan that only works from the standard start will not be there when it matters.

So build for the awkward version. Agree a meeting place that requires no phone. Write one number on paper. Decide what happens if you cannot reach each other for a day. It takes half an hour, it feels slightly absurd while you are doing it, and it is the only part of a plan that survives contact with an actual bad day.