How to prevent roaming in the border regions
Because of the physical propagation characteristics of radio frequencies, which do not respect national frontiers, it is possible that in the border regions of Switzerland, mobile equipment with a Swiss SIM card (smartphone, tablet, laptop, etc.) may switch to a foreign mobile telephone network. In order to avoid high costs as a result of this type of involuntary roaming on foreign networks, customers should switch off the “Roaming” function in their settings.
Involuntary roaming can essentially be prevented by blocking the roaming function. Depending on the specific operator, for example, data roaming can be blocked as standard when taking out a contract or buying a pay-as-you-go product.
If, despite exercising caution, a nasty surprise comes through the letter box in the form of a high bill, it is advisable to contact the respective operator or to initiate an arbitration procedure with Ombudscom (https://de.ombudscom.ch/ [Only available in German, French and Italian]).
On their respective websites, Swiss mobile telephone operators offer their customers a summary of current charges for making telephone calls using a foreign mobile telephone network.
An overview of the three operators can be found here: