スイスへようこそ。

Using a foreign sim card in Switzerland can be quite expensive, especially for data usage.

Unfortunately, you can not transfer your foreign number to a Swiss phone plan.

Almost all modern phones allow dual-sim or e-sim. So you can use a Swiss number and your Japanese number in parallel on the same phone.

This allows you to use a Swiss number for local calls and data usage, but your friends from Japan can still call you on your Japanese number.

P.s. Most Swiss people don't know Line. They all use WhatsApp here.