What video conferencing software do you recommend?

Question

Distance learning

We train employees in the office.
Task: a program like TrueConf for employee training.
1. free or with a test period of at least 20 people training.
2. no subscriptions and work exclusively in the LAN network (no external cloud services)
3. code highlighting is desirable (we train our programmers)
4. ease of installation
Giant features are not required, basic functionality will also suit.
Well, ideally, open source is desirable.

Answer 1

Try BigBlueButton

Answer 2

partial solution but as an option will go.