WhatsApp doesn't know what you say, but knows who you call
WhatsApp implemented end-to-end encryption in 2016. This means that only you and the person you are communicating with can access your conversations (see link).
But, that doesn't mean that WhatsApp isn't keeping metadata about your calls. Much like an earlier blog post about Apple Messages, WhatsApp is retaining other information related to your use of its application, such as, the telephone number of who you called and how long you talked.
Now there is a reported decision by a US District Court judge that approved the placement of a pen register and trap and trace device to capture telephone numbers being called through WhatsApp (see link to law.com article).
Just remember that despite technological advances, no communications are absolutely private because there is always some metadata.