2020 update:
Use "Swisscom Cockpit" app (
Google Play link if you don't have yet the app), right top menu -> Call settings -> Incoming Call Forward, there you can set to use or not Combox (Swisscom voicemail) for different scenarios. Don't forget to push "Save".
Same described on Swisscom site:
https://www.swisscom.ch/en/residenti...le/combox.html