For an app that has always prided itself on security, it’s only fitting that voice calls in Telegram are encrypted.
According to the Telegram team, the security aspect of Telegram voice calls is based on the end-to-end encryption of Telegram’s Secret Chats feature. With Secret Chats, after an end-to-end connection has been established, Telegram generates an image on your device that visualizes the encryption for the chat. You can then compare this image with the one seen by the person you’re chatting with on his or her device. If the two images are the same, then your chat is indeed secured against man-in-the-middle attacks.
For Telegram’s encrypted voice calls, the key exchange mechanism is easier and perhaps more fun. Instead of comparing complex images or long passcodes, you and the other person on the line just need to compare four emojis over the phone. If the four emojis match on both ends, you can be sure that your call is secure.