Hi @Patou69, welcome to our forum!
Your question has been translated to English as this is an International forum.
If you want to block a number from calling you, you'll need to download an application from the Google Play store as the Xperia XZ1 does not have an inbuilt call blocking feature. If you don't want to download an application for this, you can add the number as a contact in your device and send all incoming calls directly to your voicemail.
To do that, open up the contacts application > add a new contact with the number that you want to "block", once you had added it and before you press save, tap the 3 dots in the top right corner and check the little box next to "Send all calls to voicemail" and the next time this person contacts you, the call will be sent directly to your voicemai.
Let me know if that helps! 
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