MS graph API don't send payload when answering to a call

I have a bot which should answer to calls, here's MS doc

Actually I can Answer a call but I get no response data or error. Only the bot respond but in documentation I should get data back as mentioned in Microsoft Graph SDK here ( api method right after initialisation):

client
    .api('/me')
    .get((err, res) => {
        console.log(res); // prints info about authenticated user
    });

Here's my function :

//before that initialisation is OK
Call.prototype.Answer = function Answer() {
  console.log("answering __________>");
  var answer = {
    callbackUri: "https://ucbot.serveo.net/api/call",
    acceptedModalities: ["audio"],
    mediaConfig: {
      "@odata.type": "#microsoft.graph.appHostedMediaConfig",
      removeFromDefaultAudioGroup: false
    }
  };
  Graph.Client.api(
    "https://graph.microsoft.com/beta/app/calls/" + this.id + "/answer"
  )
  .post(answer, (err, res) => {
      console.log(
        JSON.stringify(
          " err (answer)  " + JSON.stringify(err, null, 1),
          null,
          1
        )
      );
      console.log(
        JSON.stringify("res (answer)  " + JSON.stringify(res, null, 1), null, 1)
      );
    });
};

I think that the post request is working because I get my bot answering to call, but res is undefined

Here's how I define my workflow

function PlayWorkflow(call, event) {
  // TODO: this function should be implemented in Callflow  @Mefteh.
  switch (event.resourceData.state) {
    case "Incoming":
      call.Answer();
      break;
    case "Establishing":
      //console.log("Establishing");
      break;
    case "Established":
      // console.log("Established");
      // call.Prompt();
      // call.SubscribeToTones();
      break;
    case "Deleted":
      console.log("Deleted");
      break;
    default:
      console.log(" default");
  }
}

Should I move answer() function istablished case ? If yes what should I put in incoming ? Because if I let it empty graph api will not pass to next states ( Establishing, esteblished)

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