Currently, the UIKit does not support calls. Now, the UIKit is open sourced so you could customize it to support calls however it would not be something Sendbird would support outside of the core SDK functionality.
Communications are encrypted with HTTPS/WSS for Chat. For Calls, given that SendBird Calls implements WebRTC, it uses Datagram Transport Layer Security over Secure Real-time Transport Protocol (DTLS-SRTP) to relay media. Since the media packets are encrypted between clients, not even the relay server can decrypt the media packet throughput.
This is a bit of a broad topic. You would need to be more specific about what aspects, and what languages/platforms you’re using.