Which messenger to choose for corporate communication?

Question

Messengers

Colleagues, advise a messenger for enterprise-level communication.
The main tasks are
1) the presence of rooms / groups
2) group audio and video calls
3) the ability to connect colleagues from other organizations (respectively, and the domain)
4) the presence of clients for mobile and desktop devices
5) as a plus the possibility of placement on premises
6) preferably not very high purchase/ownership cost

Answer 1

https://slack.com/
It seems to suit you.