You can choose the provider you use for making calls on your analogue fixed network connection. This is separate from your telephone line provider. This means that the line can still be billed through Swisscom but your calls are made through another (unwanted) provider. Unfortunately, there has been a recent escalation in aggressive methods to sign up new customers (such as cold calling). but you can respond to this proactively.
You can call the freephone number 0868 868 868 any time to check the provider for your connection.
If your connection is not with the provider you want, proceed as follows: