Join the call in NC Talk mobile client

Hi all!
I faced with the next problem. We host own NC server and use NC Talk for conversations and calls (all soft have latest versions). On desktop in browser all works fine. On mobile phones (we use pure AOSP + NC Talk from F-Droid) I can’t join to voice call with my friend. I enter into chat, see the record that my friend started call, but there is no green button or something similar to join call. I have tired to press Voice call button on top, but it starts a new call and don’t join to existent.
Have anyone the same problem? How to resolve it? Or what should we do to join voice call?