To start a Voice Chat in a group, please do the following:

0. Make sure your app is updated to 7.3 or higher. Click here for instructions on updating.
1. Open group profile (open group, then tap on its name or picture).
2. Tap on the three dots button in the top right corner of the screen. (See Picture 1)
3. Tap on "Start Voice chat" item in menu. (See Picture 2)

Please note! Only group owner or admins with Manage Voice Chats permission are allowed to start a voice chat in group.

Read more details about voice chats in official Telegram blog.