Voice Calls
Both private voice calls and group voice calls are currently supported by third-party, external libraries that integrate with Electrogram.
Libraries
There are currently two main libraries (with very similar names) you can use:
Older implementations
An older implementation of Telegram voice calls can be found at https://github.com/bakatrouble/pylibtgvoip (currently outdated due to the deprecation of the Telegram VoIP library used underneath).