Think twice before using the fixed line simultaneous ring option to your mobile phone.
IF you select the fixed line simultaneous ring option to ring your mobile phone. AND IF your mobile phone is out of battery. THEN incoming calls to the fixed line are immediately diverted to combox.
In other words, the status of your mobile phone dictates whether or not your fixed line works!!!
I don't have enough curiosity/patience to check what happens if the mobile is in airplane mode or is currently making a call or is SIM-locked or is not in signal range or ... I changed from simultaneous ring to diverting to mobile if the fixed line is not answered in 20 seconds.
Note that there is a bizarre exception: IF you select the fixed line simultaneous ring option to ring your mobile phone. AND IF your mobile phone is out of battery. AND IF you have multiple simultaneous call capability on the fixed line (normally yes). AND IF you are actually making a call on one of those fixed lines. THEN incoming calls to the fixed line work as normal!!! ...so if you call Swisscom support on your fixed line, any tests they try will NOT show the unwanted behaviour and Swisscom support will assume you are insane.
A further oddity: when this happened to us earlier this week with my mobile battery dead and several inbound fixed line calls flipped to combox, we tried some tests.
My wife called the fixed line from her mobile and I tried to pick up the call on the fixed line handset (HD Montreux) immediately ringing started: the handset rebooted every time (reboot = blank screen -> Swisscom logo -> zero date & time -> back to normal). Spooky!!!
I am trying to cancel Fixed network to go and after using chat and then calling Swisscom I am still unable to do this. I have the latest internet box, three Davos phones and iPhone X. I do not want my fixed line calls to ring on my iPhone. Can you please assist?