Head to the following link:
https://tk-portal.itc.rwth-aachen.de

Click on the 'call forwarding' option. This will open a new window.

This window gives the option to set up forwarding under the following conditions:

• 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 box or target number).

Selecting the option 'call not received', you can also select a time limit,
after which a given call will count as 'not received'

The four second steps for this option are roughly equivalent to one ring of the phone.