Telegram Messenger is a communication app that allows you to create encrypted chats and phone calls with other users over the Internet. A researcher has shown that in its default configuration it would allow a user’s IP address to be leaked when making call. This is caused by a default setting in Telegram, which causes voice calls to be made over P2P. Users can prevent this by changing the setting at Settings -> Private and Security to Never or Nobody. The issue has since been fixed in the 1.3.17 beta and 1.4.0 versions of Telegram for Desktop.
Source: https://www.bleepingcomputer.com/news/security/telegram-leaks-ip-addresses-by-default-when-initiating-calls/