When you call somebody or accept an incoming call in Yandex Messenger for the first time, the app or your browser requests access to your microphone and camera (if it's a video call). You can grant access right away or do it manually later. Unless you do that, you won't be able to make calls using Yandex Messenger.
How to allow access manually
On your computer
On your phone
In a browser
Windows 10
Camera access:
Click → → Privacy.
In the menu on the left, select Camera.
If the Allow apps to access your camera option is disabled, enable it.
If the Camera option is disabled in the list of apps that can access your camera, enable it.
Microphone access:
Click → → Privacy.
In the menu on the left, select Microphone.
If the Allow apps to access your microphone option is disabled, enable it.
If the Voice Recorder option is disabled in the list of apps that can access your microphone, enable it.
Tip
Additionally, make sure Yandex Messenger is added to the antivirus exception list.
macOS
In the system settings, make sure Yandex Messenger has: