Yolla requires microphone permission so the other person can hear you when you make a call.

Make sure the Yoll app has permission to access the microphone on your device.

Go to your System Settings > Apps > Yolla. Tap “Permissions”. 

Here you’ll see a list of permissions for the Yolla app.
Please check that the Microphone is enabled. Then go back to the Yolla app and try calling again.