The ringback tone behavior of IP Office systems has changed for IP Office R11.0 and higher.
After sending an INVITE request, if the IP Office receives an 18X response with SDP, it starts playing remote ringback tone. Prior to R11.0, if it then receives an 18X response without SDP, the IP Office would continue playing remote ringback tone. For R11.0 and higher, following the 18X without SDP, the IP Office now switches to local ringback tone.
In summary:
The IP Office sends an INVITE.
The IP Office receives 18X with SDP. The IP Office plays remote ringback tone.
The IP Office receives 18X without SDP:
Pre-R11.0: Continue playing remote ringback tone.
R11.0+: Switch to playing local ringback tone.
This feature is supported regardless of whether provisional response reliability (PRACK/100rel) is enabled or not.
When SIP call signaling transitions from remote to local ringback, the IP Office hosting the SIP trunk play the local ringback tone to the other end (phone or trunk).
Ringback Tone with Early Media
A special case applies for SIP trunks configured to use p-early-media. For 18x responses with or without SDP to be considered, a p-early-media header must be present in the response. If otherwise, the message is not considered with regards to early media (the system continues playing either local ringback or remote early media).
For example: The IP Office receives a 183 response with SDP and a p-early-media header with a sendonly or sendrecv parameter. The IP Office then receives a 183 response (with or without SDP):
Example 1: If the response does not include a p-early-media header, the IP Office continues listening to the remote early media.
Example2: If the response includes a p-early-media header with an inactive parameter, the IP Office switches to playing local ringback tone.
