Adopted from https://opensource.com/alternatives
Wire
Developed by the audio engineers who created Skype, Wire features secure screen sharing, file sharing, and group chat, administrator management, and the ability to switch between accounts and profiles (work and personal, for example) at will from within the app.
Wire is open source under the GPL 3.0 license and is free to use if you compile it from source on your own server. A paid option is available starting at $5 per user per month (with large enterprise plans also available).
https://wire.com/en/
Jami
Audio and video calls are made through the Jami app, available for GNU/Linux, Windows, and MacOS desktops and Android and iOS mobile devices. You can communicate using either a unique user ID (which the Jami app randomly generates the first time it's launched) or over SIP. You can use your ID and SIP in parallel, switching between protocols as needed, but you must register your ID on the blockchain before it can be used to make or receive communications.
https://jami.net/
Riot
is not just a video-conferencing solution—it's team-management software with integrated group video/voice chat communications. Communication (including voice and video conferencing, file sharing, notifications, and project reminders) happens in dedicated "rooms" that can be organized by topic, team, event, etc. Anything shared in a room is persistently stored with access governed by that room's confidentially settings. A cool feature is that you can use Riot to communicate with people using other collaboration tools—including IRC, Slack, Twitter, SMS, and Gitter.
You can use Riot in your browser (Chrome and Firefox) or via its apps for MacOS, Windows, and Linux desktops and iOS and Android devices. In terms of infrastructure, Riot can be installed on your server, or you can run it on Riot's servers. It is based on the Matrix React SDK, so all files and data transferred over Riot are secured with Matrix's end-to-end encryption.
https://about.riot.im/
Signal
For mobile devices running Android or iOS, the open source Signal app offers end-to-end encrypted voice, video, text, and photos, and it's been endorsed by security and cryptography experts including Edward Snowden and Bruce Schneier and the Electronic Frontier Foundation. Registration is simple: you're identified by your phone number, so all you have to do to register is to use the app. There's a desktop client available, but only for text chat, so it's clear that Whisper Systems, the nonprofit that maintains Signal, intends for this to be a mobile application. It's a perfect marriage between the two clients: you can use the desktop client as you work for brief messages and questions, and pick up your mobile (as you do) when you need to do a quick voice or video consultation.
https://signal.org/
Linphone
is a VoIP (voice over internet protocol) communications service that operates over the session initiation protocol (SIP). This means you need a SIP number to use the service and Linphone limits you to contacting only other SIP numbers—not cellphones or landlines. It's easy to get a SIP number—many internet service providers include them with regular service and Linphone also offers a free SIP service you can use.
With Linphone, you can make audio and HD video calls, do web conferencing, communicate with instant messenger, and share files and photos, but there are no other screen-sharing nor collaboration features. It's available for Linux, Windows, macOS desktops, and Android and iOS mobiles.
http://www.linphone.org/
Nextcloud
provides you with you own private cloud space with plenty of tools geared toward collaboration. After installing Nextcloud on a server, or opening a hosted account, you can add users who can log in and use the Nextcloud interface as a shared workspace. Users can store and share documents and files using either the web interface, or just by dropping them into a Nextcloud directory on their desktop, which gets synchronized automatically with the Nextcloud desktop client. The web interface also features a chat application called Talk, which features the ability to make voice and video calls.
Nextcloud is a shared work space, so once it's installed and configured for your organization, it can probably be the primary virtual office space for your team. It doesn't necessarily have every tool your team needs, but it's an ideal space to share and manage the data your team works on, and to coordinate remote work.
https://nextcloud.com/