Teams incoming conference call notification showing "Unknown user"

Question

Hello,

I am making an iOS app for group call.
the problem is, it always shows "Unknown User (Conference call)" on the incoming call notification (calling by a bot). here is my request code (JSON body) below:

let json = [
            "@odata.type": "#microsoft.graph.call",
            "callbackUri": "https://bot.contoso.com/callback",
            "targets": [
              [
                "@odata.type": "#microsoft.graph.invitationParticipantInfo",
                "identity": [
                  "@odata.type": "#microsoft.graph.identitySet",
                  "user": [
                    "@odata.type": "#microsoft.graph.identity",
                    "displayName": "Sender",
                    "id": VoiceCallManager.targetUserIdTX
                  ]
                ]
              ],
              [
                "@odata.type": "#microsoft.graph.invitationParticipantInfo",
                "identity": [
                  "@odata.type": "#microsoft.graph.identitySet",
                  "user": [
                    "@odata.type": "#microsoft.graph.identity",
                    "displayName": VoiceCallManager.targetUserName,
                    "id": VoiceCallManager.targetUserIdPersonal
                  ]
                ]
              ]
            ],
            "requestedModalities": [
              "audio"
            ],
            "mediaConfig": [
              "@odata.type": "#microsoft.graph.serviceHostedMediaConfig"
            ],
            "tenantId": AuthenticationManager.instance.tenantId
        ] as [String : Any]

Can you please let me know the cause of it?
Thank you.