If it's not opensource (so you can check the code for trackers) and you can't change servers (so they can't track your IP, location etc), it's not private.
Check out what What's app does, which uses the same Signal protocol lots of people love. Emphasis on "WhatsApp may start collecting..."'
> What data does WhatsApp disclose in response to government requests?
> WhatsApp scrutinizes every government request and produces only the information that is narrowly tailored to respond to each request. Depending on the request WhatsApp’s response may include, if available, basic subscriber information (such as their name, service start date, last seen date, IP address, device type, and email address), and account information (such as a user’s "about" information, profile photos, group information and contacts list). In the ordinary course of providing its service, WhatsApp does not store message logs once the messages are delivered or transaction logs of such delivered messages. In order to comply with a valid legal request, such as a valid Pen Register Trap and Trace Order in the United States, WhatsApp may start collecting message logs and call logs for a particular user indicating who the communication was to or from, the time it was transmitted and from which IP address, and the type of communication (such as a text or call).