Mattermost is the main alternative to Slack that covers most of your requirements. I have not looked at it in a while, but you will also find a few useful others here in previous questions:
Team chat with private rooms, file upload and link previews (alternative to Slack)
Team chat with native Android/iOS apps, all open source (alternative to Slack)
I would suggest using an Emoji App + Riot, or any other FOSS chat app.
PS: If you are so concerned about privacy I would switch to Linux. It is just as easy to use as windows, and it has a better support community. I would recommend Majaro Linux's KDE Plasma Edition (that's what I use)
Oh, I almost forgot to mention that You still can't turn spyware off ...
BigBlueButton would meet your requirements:
Does not require to own a server or domain (by me): While you can self-host it, there are providers offering it for you. Though BBB itself being open source, that would most likely mean to pay for the (hosting) service.
Conference Calling: Yes.
Screen Sharing: Yes. Combining the two: I've ...
Discord is free and the most popular team communication app for gamers. The platform provides users with familiar Slack-like UI, Skype-like video, and voice chats. Discord is amazing to use while participating in a Dota 2 battle, but it also meets collaboration challenges many companies face.
The tool divides channels by text and voice. ...
Have you considered using IRC? There are numerous clients for both desktop Linux(personally I use XChat, but that's far from the only option) and smartphones, it's free and doesn't require a phone number.
Your soft requirements: Most clients and servers are open source, and while by default it's not great when it comes to privacy, you can set up your own ...
We are using Openmeetings as our video conference, some what same as BigBlueButton (already suggested in one of the answer), here we can share our PDF, Spreedsheet or any other file as on whiteboard.
We can draw on whiteboard also.
I would recommend Openmeetings for this.
Here are some demo links:
https://om.alteametasoft.com:8443/next #Next version of ...
There are some alternative to Zoom:
Google Hangouts (https://hangouts.google.com/)
Google Meet (https://meet.google.com/). I prefer this one.
As mentioned in the question, that organisation has stopped using Zoom for security reasons therefore I will recommend Microsoft Teams.
Microsoft Teams allows hand raising, file sharing and storage, collaborate live in real time to work on same document. It has recently integrated whiteboard and breakout room functions also. Both free and paid plans are ...
you can do the following things with jitsi
Share your desktop, presentations, and more Invite users to a
conference via a simple, custom URL
Edit documents together using Etherpad
Pick fun meeting URLs for every meeting
Trade messages and emojis while you video conference, with integrated chat.
Try using https://ricochet.im/. It actually does meet 3, because of these interesting bits of Unicode called emojis. On Windows 10, you can pick emojis by pressing Win+., There are more than 3K of them, so hopefully that qualifies as "loads".
(Sorry, this should be a comment, but I don't have enough rep.)
With https://whereby.com/ you can provide video meetings in the browser. Your attendees just need to visit your custom link to whereby.
You can also share your screen and for small teams (up to 4 attendees) it is gratis.
This is not a code library but a software recommendation. Anyway, i hope this helps.
If you are looking for software (self-hosted):
You should provide more information about the platform you're using. There are lots of solutions that exist for this. Here is one for WordPress. There are more for Django, Ruby on Rails, JavaEE, etc.
Your platform is important because, unless you want to use some innovative blockchain like Bitmessage (in which ...
Telegram has a Bot API that does exactly what you seek.
They have samples for all the possible programming languages, so it's totally not a problem.
Going through your requirements:
offers an api to integrate in my development environment
offers a web application to use within a browser
it should be easy for me to add new users to my messaging ...
Jitsi Meet is open source, does not require any account and you can self-host it if you want. It works in the browser, so no installation needed.
Jitsi Meet - Secure, Simple and Scalable Video Conferences that you use as a standalone app or embed in your web application.
You can try it on https://meet.jit.si/. There are other instances as well, you'll find ...
You can use Zoom:
supports 1 on 1 chats (the admin of the meeting can require a password to access the meeting)
does not require any logins
has text chat
has screen sharing
free tier: the Zoom Basic (free) plan has a 40-minute time limit on meetings with 3 or more participants.
FYI: Zoom has been criticized for its data collection practices.