I was looking for the same as the original poster. The currently working solution is described here:
https://www.swisscom.ch/en/residenti...le/combox.html

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#21#: Forward all to calls to your COMBOX®:
#61#: Forward to your COMBOX® if you don’t take the call
#67#: Forward to your COMBOX® if you are on a call.

Switching off your COMBOX®
Proceed as follows to switch off your COMBOX®:

1. Enter the respective key combination on your telephone: #21#, #61# or #67# – depending on which forwarding mode you want to switch off.
2. Press the Call button.
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