After you to receive/make calls, you can start taking inbound calls.
Click the Accept button to accept the call. The ongoing call appears as shown below:
You have controls to mute/unmute calls.
If you're using Rocket.Chat's enterprise edition you can hold and unhold acording to your workflow needs.
After the call ends you can leave your notes in the call wrap-up. It is an enterprise-only feature.
Enterprise edition also offers you the flexibilty to switch device during an ongoing call.