Today’s invention is a software plugin for networked diaries.
When you set up your out-of-office message, the programme would automatically ask you for someone to handle your high priority messages. It would then detect if the person who is nominated to take those urgent calls (Ms A, say) has themselves booked a holiday while you are away.
If they have, then you would be informed and any emergency calls would be automatically redirected to Mr B (as nominated by Ms A) during that period.
The depth of this redirection sequence would be limited so that the ultimate call handler still had some idea of what to do.
In particular, this approach would also ensure that no loops could form ie
MsA->MrB->MsC->MsA would be ruled out.