After you start a connection with a lead or client, you'll see the following:

The contact's first and last name will be shown if it was added earlier or came from your integrations. Between the phone number and the name is an External link icon that opens this contact's page in your CRM or shop. This is useful when a customer wants fast help — you can open the CRM link and see all the details you need right away.
You can click the mic and receiver icons to mute or adjust speaker and microphone levels. There is also a Voice drop button to leave a prerecorded message on the receiver's voicemail. Use the history button to view past activity for the number, and use the keypad for tone input to navigate IVR systems.
To end the call, press the red receiver icon at the bottom of the add-on window.