As much as I love the swiss mobile network - saying that it works everywhere is wrong. In the mountains you still have smaller regions without service from all major providers. Especially when you are high up, you have to know that you cannot rely on 100% on your mobile connection.
You can take a look at the Swissom mobile coverage map here (german):
https://scmplc.begasoft.ch/plcapp/pages/gis/netzabdeckung.js...