All of us know that exists 3 kinds of Voip security mode…
Unsecured, Sip Secured and Secured.
When will create an UM dial plan, we need to set a specific mode.
By default, the Unsecured mode is selected when we’ll create it. In this mode, Client Access and Mailbox servers associated with this UM dial plan send and receive data without encryption. Neither the RTP media channel nor SIP signaling information are encrypted.
When you choose SIP secured, the SIP Signaling is encrypted, RTP uses TCP, but in this time, not encrypted. In this case, the MTLS is used to encrypt the SIP signaling traffic and VoIP data
In the Secured mode, both SIP signaling and RTP are encrypted. The secured secured signaling media channel uses SRTP and SIP signaling uses MTLS.
Reference:
https://technet.microsoft.com/EN-US/library/bb123819(v=exchg.150).aspx
