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.

Mattermost is an open-source primary self-hosted team communication service aimed at developers but suitable for all kinds of teams.

Stackhero.io is a France-based managed cloud service provider that offers a managed instance of Mattermost Cloud.

atolia is a team communication service that offers the typical features chat, audio and video calls as well as additional features like shared calendars and project management.

Wire is an team communication service from Switzerland with Apps for iOS, Android, Windows, macOS and Linux. The communication is end-to-end encrypted and the apps are open source.

ginlo Business is a team communication service from ginlo. ginlo is a company from Germany that also offers ginlo Private, a instant messager for private customers.

Private Discuss is a team communication service from France with a stron focus on audio/video conferences. They also offer instant messaging and apps for all major platforms.

Element is an open source app for team communication. It is end-to-end encrypted and uses the open Matrix protocol. The decentralized Matrix protocol enables communication with Matrix servers and people with other Matrix clients.

Mattermost is an open-source team communication service aimed at developers but suitable for all kinds of teams. It can be selfhosted and offers app for all major platform.

Rocket.Chat is a open source team communication service. They offer many guides for self-hosting on different platforms (Docker, Kubernetes, ect.) and clients for Android, iOS, Windows, macOS and Linux.