You can start voice and video calls in individual conversations in the app. If you want to talk to multiple users at the same time, use video conferencing. During a call, you can hang up and mute yourself. Incoming calls can be rejected.
The "Voice and Video Calling" module must first be activated by the administration. Read Activate additional tools (Calender, surveys, jitsi-videoconferences and more).
You must have user permission to make calls in order to make calls yourself. Without this permission, you can still receive calls. As an admin, find more information here: Granting users permissions for additional modules.
Users can suppress all incoming calls in their privacy settings. In this case, no call can be initiated with this user and an error message will appear. For more information, see Suppress calls.
In the web/desktop client
If you want to edit the video and audio inputs and outputs, read the article Additional user settings | Call settings.
Click on the desired individual conversation and then click on the 
If you want to call a user you have not contacted before, go to the contact book and open the user's profile page to start a call. For more information, see: Contacts and profiles.
In iOS
Go to the desired individual conversation and tap the circled 
If you want to call a user you haven't contacted before, you can alternatively start calls from the user's profile page. For more information, see: Contacts and profiles.
On Android
Go to the desired individual conversation and then tap the
If you want to call a user you haven't contacted before, you can alternatively start calls from the user's profile page. More information here: Contacts and profiles.