In Sunny day, the following conditions must be met for the call signalling to negotiate the use of SRTP between IP Office and the HQ user’s phone:

• Signalling is done on the SM Line over TLS
• Regardless of whether Communication Manager is configured for Initial Direct Media or IP-IP audio (shuffling), signaling and Media Encryption should be configured consistently on Communication Manager, the central Communication Manager Media Gateway, Session Manager, IP Office, and the Centralized SIP phone.

IP Office negotiates SRTP separately for each call leg. For example, SRTP is disabled for the endpoint of the local IP Office user, or the IP Office user uses an endpoint type in which SRTP is not supported. In such a case, the media flow between IP Office and the IP endpoint of the IP Office user will use RTP.

This call flow scenario has two legs, as described in the following table.