I am trying to initiate a call between two users using the Microsoft Graph API, but I am encountering an error.

Request Body:

{  
  "@odata.type": "#microsoft.graph.call",  
  "callbackUri": "url",  
  "source": {  
        "@odata.type": "#microsoft.graph.participantInfo",  
        "identity": {  
            "@odata.type": "#microsoft.graph.identitySet",  
            "user": {  
                "@odata.type": "#microsoft.graph.identity",  
                "displayName": "User1",  
                "id": "123456  
            }  
        }  
    },  
  "targets": [  
    {  
      "identity": {  
        "user": {  
          "displayName": "User2",  
          "id": "123456"  
        }  
      }  
    }  
  ],  
  "requestedModalities": [  
    "audio"  
  ],  
 "mediaConfig": {  
    "@odata.type": "#microsoft.graph.serviceHostedMediaConfig"  
  },  
  "tenantId": "55666655"  
}  

I get this message as reponse :

{  
    "error": {  
        "code": "2255",  
        "message": "Call source identity invalid.",  
        "innerError": {  
            "date": "2025-01-18T16:58:56",  
            "request-id": "12345",  
            "client-request-id": "12345"  
        }  
    }  
}  

Issue Details:

I can successfully initiate a call between a bot (application) and a user when the source type is "application". However, when I try to initiate a call between two users (as in the request body above), I receive the "Call source identity invalid." error.

Question:

Is it possible to initiate a call between two users using Microsoft Graph API? If so, what am I missing in my request?

Any insights or alternative approaches would be greatly appreciated.