If you are calling within Switzerland, you have excellent coverage inside any building with a WiFi connection.
If you call or send a message from abroad, you avoid roaming costs: calls and SMS messages sent via WiFi Calling are charged at the same rate as in Switzerland.

To ensure that the phone does not connect to the mobile network, we recommend turning on flight mode first and then activating WiFi.

You can find a list of all WiFi Calling compatible devices here.