In case of connection issues, it may be useful to get a screenshot of the call screen for the Telegram developers. Here's how to get the call debug info screen in the Telegram apps. To save your time, only send it if Telegram support asks for it:
- iOS & Android: Tap 10 times on the name of the call screen
- Telegram Desktop/Lite: CTRL+CLICK (or CMD+CLICK on a Mac) on the call screen. It works only when you're in debug mode.
- Telegram macOS (native): not possible
It could be that a local firewall or other system setting is causing issues. Makes sure to check if your network allows the following ports:
- UDP 520-550 (in and outgoing)