Calls in Session are end-to-end encrypted and offer a good level of privacy. Unlike messages (which use onion-routed networking), the current implementation of calls uses peer-to-peer networking. This means your IP will be shared with your call partner as well as an OPTF operated STUN/TURN server. Although this is acceptable for most people, you should always make sure to assess your own personal situation to determine whether the risk of exposing your IP is worth it. If you're in an extremely high-risk situation, we do not recommend enabling peer-to-peer calls — onion-routed calls are on the way.
In order to prevent spam and protect privacy, you can only send and receive calls with people in your contacts list — not unknown Session IDs or people in your message requests.