Hello @Sergei Zinchenko ,
Please have a look to know about latency to expect for different resources.
Also the scenario that you are looking for is not possible with the subscription. callRecord subscription works in a way to get the call details when the call ends. Also, it didn't work in real-time and for all user joining/disconnecting the call.
I would suggest you to go raise a user voice for it - https://microsoftteams.uservoice.com/
Thanks,
Hunaid Hanfee
If the response is helpful, please click "Accept Answer" and upvote it. You can share your feedback via Microsoft Teams Developer Feedback link. Click here to escalate.