title: Microsoft Graph: Requests to voice calls endpoint return 500 UnknownError without message
body: <p>I created an app for Microsoft Teams which provides a bot and makes requests to the Graph API to get some user details.
  All is working fine so far. The app is trusted and I can work with the Graph API.</p>
  
  <p>I set up the bot to support voice calls and when I call the bot, I do get a POST request with all information.
  However when I want to <a href="https://docs.microsoft.com/en-US/graph/api/call-reject?view=graph-rest-beta&amp;tabs=http" rel="nofollow noreferrer">work with this call, e.g. reject</a> it immediateley, the request to Microsoft Graph API fails with status code 500: UnknownError.</p>
  
  <p>This is my testcode:</p>
  
  <pre><code>async RejectIncomingCall(id) {
    const client = await getAuthenticatedClient(this.token);
  
    try {
      return await client.api(`/app/calls/${id}/reject`).version('beta').post('');
    } catch (e) {
      console.log('Error rejecting call!', e);
    }
  }
  </code></pre>
  
  <p>I have no information what's wrong.
  I can reproduce this behavior using the Graph Explorer when I try to <a href="https://docs.microsoft.com/en-US/graph/api/call-get?view=graph-rest-beta&amp;tabs=http" rel="nofollow noreferrer">get a call</a>:
  <a href="https://i.stack.imgur.com/QpUy2.png" rel="nofollow noreferrer"><img src="https://i.stack.imgur.com/QpUy2.png" alt="enter image description here"></a></p>
  
  <p>So in fact all requests to the call API seem to be failing.<br>
  Any idea, what I did wrong?<br>
  Did I missunderstood something or is the beta api at this point just not working?</p>