Q: Hearing problem

See if the troubleshooting information in this support document helps. If you can't hear a person on a call or voicemail or if the sound isn't clear on your iPhone - Apple Support

And the support document that I provided you is for troubleshooting if you cannot hear anyone.

Siri uses the speaker. Have you checked to make sure your speaker volume is turned up? If so, you may need to make a Genius Bar appointment to have the device examined.

Considering that you purchased the device in Switzerland, that is where the service would have to be. I'm sorry I missed in your original post about where it was purchased and where you live. iPhone warranty work is not international, and is country specific. How you handle it at this point is your decision. To get Apple's assistance you would need to bring the device to Switzerland for service, or risk voiding all support from Apple.