In the answers to FAQs on the official website of the Telegram came new item.
"Telegram uses the camera or microphone in the background."
What is known: — The app will use the microphone, if can be minimized during a call, record a voice or video message. — On some devices, the application requests information about the camera, thus there is a request (taken at this moment not). — Anyone can check whether the app downloaded from Google Play or AppStore that code that is posted on GitHub.