Give members a role while they sit in a voice channel, and take it off when they leave. Use it to open a text channel only to the people in a call, or to show who is in voice right now. (Configure → Voice-channel roles.)
Each rule names one role and where it applies: Any voice channel (every voice and stage channel except your AFK channel), or Specific channels or categories. A category covers every voice channel inside it, including the rooms a dynamic voice hub makes there. Packet takes the role off 10 seconds after a member leaves, so a quick reconnect doesn't flicker it. Give each rule a role of its own: Packet takes it off anyone who isn't in a covered channel, even someone who got it another way. You can have up to 25 rules. Packet keeps nothing about the member, so the role itself is the only record.