The Microsoft Exchange Unified Messaging Call Router service rejected the call for the following reason

Published on Wednesday, May 11, 2022

In the process of adding an Exchange Server 2019 to a current Exchange organization with Exchange Server 2013, Auto Attendant/UM stopped working, investigating event viewer logs in Exchange Server 2013 server, I found this event entry (Event Id 1647)

The Microsoft Exchange Unified Messaging Call Router service rejected the call for the following reason: 15647;source="ex2013.contoso.net";reason="Access to Active Directory error occured." Microsoft.Exchange.UM.UMCore.CallRejectedException: The Mailbox server with the FQDN ex2019.contoso.net couldn't be located in Active Directory.
   at Microsoft.Exchange.UM.UMCore.EnterpriseRedirectionTarget.GetRoutingInformation(String serverFqdn, Boolean isSecuredCall, String& routingFqdn, Int32& routingPort)
   at Microsoft.Exchange.UM.UMCore.EnterpriseRedirectionTarget.GetBackEndBrickRedirectionTarget(ADUser user, IRoutingContext context)
   at Microsoft.Exchange.UM.UMCore.EnterpriseRedirectionTarget.GetForNonUserSpecificCall(OrganizationId orgId, IRoutingContext context)
   at Microsoft.Exchange.UM.UMCore.AutoAttendantCallHandler.HandleCall(CafeRoutingContext context)
   at Microsoft.Exchange.UM.UMCore.RouterCallHandler.InternalHandle(ICallHandler[] handlers, CafeRoutingContext context)
   at Microsoft.Exchange.UM.UcmaPlatform.UcmaCallRouterPlatform.TryHandleIncomingCall(CallReceivedEventArgs`1 args, Exception& error, Boolean& isDiagnosticCall)
   at Microsoft.Exchange.UM.UcmaPlatform.UcmaUtils.<>c__DisplayClassf.<ProcessPlatformRequestAndReportErrors>b__d()

The error might be misleading as it implies some Active Directory connection issue, but it was mentioning the newly incorporated Exchange Server 2019 and I thought, why?

I then remember I migrated System Mailboxes to Exchange 2019 box, especially, these two, that have UM-related capabilities:

Name                  : SystemMailbox{bb558c35-97f1-4cb9-8ff7-d53741dc928c}
PersistedCapabilities : {OrganizationCapabilityUMGrammarReady, OrganizationCapabilityPstProvider,
                        OrganizationCapabilityMessageTracking, OrganizationCapabilityMailRouting,
                        OrganizationCapabilityClientExtensions, OrganizationCapabilityGMGen,
                        OrganizationCapabilityOABGen, OrganizationCapabilityUMGrammar}

Name                  : SystemMailbox{e0dc1c29-89c3-4034-b678-e6c29d823ed9}
PersistedCapabilities : {OrganizationCapabilityUMDataStorage}

Boom! that might be it... so I moved them back to Exchange 2013 mailbox database and AutoAttendant/UM started to work again.

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