On your mobile
Type this code like a phone number and press call:
**004*<number>**20#
Check: *#61#, turn off: ##004#. If your phone doesn't accept the code, look in your provider's app for „call forwarding when not answered“ (Rufumleitung bei Nichtmelden).
Landline with FRITZ!Box
Open fritz.box, then Telefonie, Rufbehandlung, Rufumleitung, Neue Rufumleitung: all calls, target your assistant number, delayed 20 seconds, apply.
Other landline (Telekom, Vodafone, o2 …)
In your customer portal under „Anrufweiterschaltung“ set the option bei Nichtmelden (when not answered) to your assistant number.
Forwarding kicks in when you don't pick up for 20 seconds, are busy or your phone is off. It costs like a call to a German landline – included in most flat rates.
Questions? WhatsApp Vlad in English