Fleep is a team communication service from Estonia that offers many integrations and Apps for iOS, Android, Windows, macOS and Linux. They don't offer audio and video calls directly in their application, but they do offer integration with an external tool, so it's easy to start a call from the chat window.
Fleep offers a free plan (called Basic) with a little less features and the restriction that each user can only create one group chat. In contrast to slack, all old messages can be read in the free plan.
The first paid plan costs 5€/user/month and includes almost all features like team-management, unlimited groups and 100GB storage.
There is also an Enterprise plan which is only available on request.
Read more on the pricing page of the service.