Important: most are team tools, not consumer calling apps
Rocket.Chat, Wire, and Nextcloud are aimed at organizations, with administration, SSO, and compliance features. Element and Twake Chat are Matrix messengers that individuals and groups can use. Calling regular phone numbers, which Skype supported, is not a simple built-in feature of these tools and needs separate telephony integration.
Closest to Skype
Element is closest for secure messaging and calls, with end-to-end encryption, voice and video, and federation across servers on the Matrix protocol. Wire offers encrypted messaging, calls, and file sharing, with a polished consumer-style experience. Nextcloud adds chat and video calls through its Talk app alongside files and calendars.
License details
Rocket.Chat is MIT outside its ee directory, which has a separate enterprise license. Element is multi-licensed under AGPL-3.0, GPL-3.0, or a commercial license. Nextcloud and Twake Chat are AGPL-3.0, and Wire's server is AGPL-3.0 while its client apps are GPL-3.0.
Encryption and federation
Element and Twake Chat use the Matrix protocol, which supports end-to-end encryption and federation between independent servers, plus bridges to other networks. Wire uses always-on end-to-end encryption. Rocket.Chat offers optional end-to-end encryption, and Nextcloud encrypts calls in transit with optional end-to-end encryption for some features.
Maintenance status
Rocket.Chat, Nextcloud, Element, Wire, and Twake Chat all had commits within days of this update. Twake Chat has a small community compared with the others.