Hi @MarekAdamek and welcome to the support forum!
I have translated your question to English as this is an International forum, please post any replies in English and it will be easier for me and other users to assist you.
It is most certainly possible to make and take part of conference calls in Xperia XZ1 Compact. During an ongoing call, Simply tap Add call on the left side, below the number or name of the caller and you should be able to add another number by either entering a number or choosing a contact from your contact list or call log.
I hope that helped! Let me know if you have any further questions!
