index.html

<html>
  <head>
  </head>

  <body>
    <h1> WebRTC Voicemail </h1>
    <button onclick="window.startVoicemail()"> Start Voicemail </button>

    <script>
      let pc = new RTCPeerConnection()
      navigator.mediaDevices.getUserMedia({audio: true})
        .then(stream => {
          stream.getTracks().forEach(track => {
            pc.addTrack(track, stream)
          })
          pc.createOffer().then(d => pc.setLocalDescription(d))
      }).catch(window.alert)

      function startVoicemail() {
        fetch('/create-voicemail', {
            method: 'POST',
            headers: {
              'Content-Type': 'application/json'
            },
            body: JSON.stringify(pc.pendingLocalDescription)
        })
        .then(response => response.json())
        .then(sdp => {
          pc.setRemoteDescription(sdp).catch(window.alert)
        })
      }
    </script>
  </body>
</html>