[bargl]

I have a rule with everyone who could have an emergency, 3 calls in a row.

I know it can be burdensome, but I don't answer in an emergency but if I ever see I missed 3 calls then I know it was an emergency.

[waterhouse]

That works too. Both Apple and Android have built-in support for do-not-disturb policies that will let someone through if they call twice in a row (defined as "within 3 minutes" on Apple and "within 15 minutes" on Android, it seems).