Graph API: create a PSTN call and invite a PSTN participant

Hello everybody,

We are using the MS Graph API within a C# project.

After successful creating a PSTN call from our bot like described here https://docs.microsoft.com/en-us/graph/api/application-post-calls?view=graph-rest-1.0&tabs=http#example-7-create-peer-to-peer-pstn-call-with-service-hosted-media we run into the next problem:

We want to invite another PSTN participant to this existing call like described here: https://docs.microsoft.com/en-us/graph/api/participant-invite?view=graph-rest-1.0&tabs=http#example-4-invite-one-pstn-participant-to-an-existing-group-call.
But we receive following event:

{
"value": [
{
"changeType": "Deleted",
"resourceUrl": "/communications/calls/981f6100-cb65-42d9-af3c-1390c68cac8b/operations/5fedb136-cff9-446d-a8bc-4166db960e19",
"resource": "/app/calls/981f6100-cb65-42d9-af3c-1390c68cac8b/operations/5fedb136-cff9-446d-a8bc-4166db960e19",
"resourceData": {
"@odata.type": "#microsoft.graph.inviteParticipantsOperation",
"participants": [
{
"@odata.type": "#microsoft.graph.invitationParticipantInfo",
"identity": {
"@odata.type": "#microsoft.graph.identitySet",
"phone": {
"@odata.type": "#microsoft.graph.identity",
"id": "+49175********"
}
}
}
],
"status": "failed",
"resultInfo": {
"@odata.type": "#microsoft.graph.resultInfo",
"code": 403,
"subcode": 9999,
"message": "addParticipants failed for participant 4:+49175******. Underlying reason: Call does not contain the metadata required for making this PSTN dial out.. DiagCode: 403#10151"
},
"id": "5fedb136-cff9-446d-a8bc-4166db960e19"
},
"@odata.type": "#microsoft.graph.commsNotification"
}
],
"@odata.type": "#microsoft.graph.commsNotifications"
}

Any idea why we can not invite another PSTN participant to an existing call from the bot to a PSTN number?

Thanks.

Regards

Gerd