Help setting up internal emergency number

I need to setup an internal Emergency number eg extension 2222

In the day time/working this needs to ring two internal phones so a ring group will be fine for this.
However out of working hours this needs to forward all calls to an external number which is manned overnight. This switch over times/days could vary so ideally would need to be controlled by someone dialling a code on the phone to switch between the internal and external two destinations.

Can anyone suggest a good way of setting this up?
Running Freepbx 2.11