Matrix is my favourite, and easy to use, you just need to pick a client that supports it https://matrix.org/ecosystem/clients/ (Element is the most common) and create an account. You can also self-host if you want more privacy/control.
Signal has a desktop client, but sadly, it still needs a phone to work.
I also use Steam chat for playing.
A more direct alternative would be
- Discord -> Revolt
- Teamspeak -> Mumble
These are numbers from 2016. Is there any place where we can read these contracts? Maybe with the current situation, conditions are changing. Hopefully, people put a clause in case of animosity between countries.