Rejoining a live call always works, from the same button you joined with or from the banner in your inbox, and rejoining never double-charges your time.
If your match vanished, the room holds their place for a couple of minutes and the question timer pauses while they are away. If they cannot make it back, you can end the call. No sound? Use "Test your audio" in the room before the call: a live mic meter, and mic and speaker pickers whose choices carry into the call. Mid-call, check the same pickers and your browser's microphone permission.
The timer starts from zero only when the second soul joins, so waiting alone never uses your talk time. Avoid leaving by the back button; leaving ends the call for both of you, and the app asks you to confirm first. A call parked in an idle tab for over an hour needs the page refreshed to reconnect.