Today i have tried to start a Reversi session with my coworker (on the same Spark version). He didn't had a chat window open with me, so nothing happened at all. I thought it should be possible to start a Reversi session even if no window is already open on the other side. Just like it happens with a file transfer or a voice call.
And even if windows are opened on the both sides it just shows a Reversi icon on the other side and no Accept or Reject buttons.