Unable to use bot with graph api to call a user on teams whose user id I provided in the calls api

Expected behavior
Using bot and graph api, I want to call a user on teams whose user id I provided in the calls api

I followed this example https://docs.microsoft.com/en-us/microsoftteams/platform/sbs-calling-and-meeting

These are the steps I have done.
Registered an App in Azure using demo tenant
Created a policy for a demo tenant user for creating the online meeting on behalf of that user
Added following Graph API Applications permissions to my Azure App*

• CallRecords.Read.All
• Calls.Initiate.All
• Calls.AccessMedia.All
• Calls.InitiateGroupCall.All
• Calls.JoinGroupCall.All
• OnlineMeetings.ReadWrite.All
• OnlineMeetings.Read.All
• Calls.JoinGroupCallAsGuest.All

Granted Admin consent for the above permissions

Created a Bot Channel Registeration in Azure account which have subscription enabled.
While registering the bot, used https://<my_ngrok_url>/api/messages as the messaging endpoint.
Enabled the Teams Channel
Provided App-Name, Resource Group and other required information
I did Select the Calling tab on the Teams channel page. Select Enable calling, and then updated the Webhook (for calling) with (https://yourNgrok/callback.
I am receiving the notifications.So this part works.

For bot my manifest is

{
  "$schema": "https://github.com/OfficeDev/microsoft-teams-app-schema/blob/preview/DevPreview/MicrosoftTeams.schema.json",
  "manifestVersion": "devPreview",
  "version": "1.0.0",
  "id": "<<app-id>>",
  "packageName": "com.acs.sample",
  "developer": {
    "name": "Contoso",
    "websiteUrl": "https://example.azurewebsites.net",
    "privacyUrl": "https://teams.microsoft.com",
    "termsOfUseUrl": "https://teams.microsoft.com"
  },
  "icons": {
    "color": "color.png",
    "outline": "outline.png"
  },
  "name": {
    "short": "Calling bot",
    "full": "Calling bot"
  },
  "description": {
    "short": "Calling bot",
    "full": "Calling bot"
  },
  "accentColor": "#FFFFFF",
  "bots": [
    {
      "botId": "<<app-id>>",
      "scopes": [ "personal", "team", "groupchat" ],
      "supportsFiles": false,
      "isNotificationOnly": false,
      "supportsCalling": true,
      "supportsVideo": true
    }
  ],
  "permissions": [ "identity", "messageTeamMembers" ],
  "validDomains": [ "*.ngrok.io" ]
}

Installed Calling Bot in Teams. Installed without any error. I can make calls from the bot and see the notifications on my calling endpoints. One strange behavior is Its message text-box become disabled after a while but I can still make call from the bot.

Now I want to make call to a user from an endpoint.
From my python code. I used this code to get access token.

def getToken():
    url = "https://login.microsoftonline.com/tenant_id/oauth2/v2.0/token"
    type = 'post'
    data={
        "client_id": 'client_id',
        "client_secret": 'client_secret',
        "scope": 'https://graph.microsoft.com/.default',
        "grant_type": 'client_credentials',
    }

    resp = requests.request(
        method = type,
        url = url,
        data = data
    )
    resp_data = resp.json()
    return resp_data['access_token'];

This access token is correct because I used the token to get users list etc using graph api to see if I can use the api or not.

Then using this token I call calls endpoint

def call_user():
    accessToken = getToken();
    url = "https://graph.microsoft.com/beta/communications/calls"
    type = 'post'
    
    data={
  "@odata.type": "#microsoft.graph.call",
  "callbackUri": "https://my_ngrok_url/call",
  "targets": [
    {
      "@odata.type": "#microsoft.graph.invitationParticipantInfo",
      "identity": {
        "@odata.type": "#microsoft.graph.identitySet",
        "user": {
          "@odata.type": "#microsoft.graph.identity",
          "displayName": "John",
          "id": "user_id_to_whom_i_want_to_call"
        }
      }
    }
  ],
  "requestedModalities": [
    "audio"
  ],
  "mediaConfig": {
    "@odata.type": "#microsoft.graph.serviceHostedMediaConfig"
  }
}

    resp = requests.request(
        method = type,
        url = url,
        data = data,
        headers={"Authorization": "Bearer " + accessToken},
    )
    resp_data = resp.json()
    return resp_data

It gives internal server error as response. Please help what i am doing wrong.

Start at the Python getToken and call_user entry points, then inspect the complete status, response body, and request encoding returned by /beta/communications/calls. Compare the request with the linked calling example and verify the required Graph permissions and bot configuration; done means the endpoint returns a successful call response rather than an internal server error.