The most comfortable option for setting up call forwarding is to do it directly in the TK-Portal.

To do this, navigate to the TK-Portal and click on the slider (1) under Forwarding and then on the gear wheel (2) to the right of it. A new window will open.
In this window you have the possibilities to set up forwardings for different states: all calls calls received when line is busy unreceived calls calls during no network coverage calls while no phone is registered There are always two target options (Voice There are always two destination options (voicebox or a target number)
If you select the call not received, you can also select a time limit, after which a given call will count as not received. The 4 second steps for this option are roughly equivalent to one ring of the phone.