Microsoft Graph Api createCall throws Unknown internal server error

Question

I'm creating a Java application which create a call between participants. According to the documentation (https://docs.microsoft.com/en-us/microsoftteams/platform/bots/calls-and-meetings/registering-calling-bot) I already create a Microsoft bot and added MsTeams channel. In the Azure portal I create an App registration with required permissions and when I call a POST https://graph.microsoft.com/v1.0/communications/calls I got an error

POST https://graph.microsoft.com/v1.0/communications/calls
SdkVersion : graph-java/v5.13.0
SdkVersion : graph-java/v5.13.0
{
    "callbackUri":"CALLBACK_URL",
    "direction":"outgoing",
    "mediaConfig":{"blob":""},
    "mediaState":{"audio":"active"},
    "meetingInfo":{
        "organizer":{
            "user":{
                "displayName":"User 2",
                "id":"63678056-8b5a-47e4-8688-dc77b73ff439"
            }
        }
    },
    "requestedModalities":["audio"],
    "source":{
        "identity":{
            "user":{
                "displayName":"User 2",
                "id":"63678056-8b5a-47e4-8688-dc77b73ff439"
            }
        }
    },
    "targets":[
        {
            "identity":{
                "user":{
                    "displayName":"User 1",
                    "id":"a65a4c0b-9174-4166-b8d2-825822b4822b"
                }
            }
        }
    ],
    "tenantId":"8a61bdf8-xxxx"}

500 : Internal Server Error
client-request-id : 15893b89-16a4-4bef-9d5b-9f358c13bee1
Content-Type : application/json
Date : Tue, 15 Mar 2022 16:37:57 GMT
request-id : 1840d2a7-7b98-4206-8c2e-46a57017f67c
Strict-Transport-Security : max-age=31536000
Transfer-Encoding : chunked
Vary : Accept-Encoding
x-ms-ags-diagnostic : {"ServerInfo":{"DataCenter":"West Europe","Slice":"E","Ring":"5","ScaleUnit":"001","RoleInstance":"AM4PEPF000157B4"}}
{
    "error": {
        "code": "9999",
        "message": "Unknown internal server error.",
        "innerError": {
            "date": "2022-03-15T16:37:57",
            "request-id": "1840d2a7-7b98-4206-8c2e-46a57017f67c",
            "client-request-id": "15893b89-16a4-4bef-9d5b-9f358c13bee1"
        }
    }
}

Example of the accessToken

{
    "typ": "JWT",
    "nonce": "bA2X57jOLNLhtgSE1ymo2U_ap9IOV_tmxjuMQ5-LQpQ",
    "alg": "RS256",
    "x5t": "jS1Xo1OWDj_52vbwGNgvQO2VzMc",
    "kid": "jS1Xo1OWDj_52vbwGNgvQO2VzMc"
}.{
    "aud": "https://graph.microsoft.com",
    "iss": "https://sts.windows.net/8a61bdf8-1b33-4b6d-a95f-f99546e97f08/",
    "iat": 1647361546,
    "nbf": 1647361546,
    "exp": 1647365446,
    "aio": "E2ZgYHjGFc69hKdaxufkZIVLP7dsAQA=",
    "app_displayname": "ms-teams-test-call-app",
    "appid": "d04b6155-xxxx",
    "appidacr": "1",
    "idp": "https://sts.windows.net/8a61bdf8-1b33-4b6d-a95f-f99546e97f08/",
    "idtyp": "app",
    "oid": "1f18bac4-xxxx",
    "rh": "0.AVAA-L1hijMbbUupX_mVRul_CAMAAAAAAAAAwAAAAAAAAAB_AAA.",
    "roles": [
    "Calls.JoinGroupCall.All",
    "Calls.InitiateGroupCall.All",
    "Calls.JoinGroupCallAsGuest.All",
    "Calls.AccessMedia.All",
    "Calls.Initiate.All"
    ],
    "sub": "1f18bac4-a630-42fe-bca2-7acffb476ee6",
    "tenant_region_scope": "NA",
    "tid": "8a61bdf8-xxxx",
    "uti": "xiwA4EWhSU6hdfviSQ_HAA",
    "ver": "1.0",
    "wids": [
    "0997a1d0-0d1d-4acb-b408-d5ca73121e90"
    ],
    "xms_tcdt": 1642728719
    }.[Signature]

And one more - I can't find what information what I should set to the mediaConfig.blob if I'm using AppHostedMediaConfig?