Data packs for Switzerland are added to your account automatically as soon as you’ve used up your data. This feature is turned on by default, but you can always switch it off in your Cockpit. Go to «Settings» and «Data and services».
If you’ve deactivated this option and use up your available data, surfing speed will be reduced. But you’ll still have unrestricted access to your Cockpit. If you want to continue surfing at full speed, you can buy additional data packs there.