Explanation:

You can specify the startup mode for the Microsoft Exchange Unified Messaging service on a Mailbox server. By default, the Mailbox server will start up in TCP mode, but if you're using Transport Layer Security (TLS) to encrypt Voice over IP (VoIP) traffic, you must configure the Mailbox server to use TLS or Dual mode. We recommend that Mailbox servers be configured to use Dual as the startup mode. This is because all Client Access servers and Mailbox servers can answer incoming calls for all UM dial plans, and those dial plans can have different security settings (Unsecured, SIP secured, or Secured).
This example sets the startup mode for a Mailbox server named MyUMServer1 to Dual mode.
Set-UMService -Identity MyUMServer1 -UMStartUpMode Dual
Note: Set-UMService: The Set-UMService cmdlet is used to set the properties, including
UMStartUpMode, on a Microsoft Exchange Server 2013 Mailbox server or Exchange
Server 2007 or Exchange Server 2010 Unified Messaging server that's running the
Microsoft Exchange Unified Messaging (UM) service.