DTMF Transfer for Spiral Calls
A spiral call occurs when a call’s signaling messages loop back through the Oracle® Enterprise Session Border Controller. Most commonly the signaling path is from one UA, through the Oracle® Enterprise Session Border Controller, to a call server, back through the Oracle® Enterprise Session Border Controller, to another UA. The media path is from one UA, through the Oracle® Enterprise Session Border Controller, to the other UA. For DTMF indication processing, only the call legs between the endpoints and Oracle® Enterprise Session Border Controller are considered.
The Oracle® Enterprise Session Border Controller evaluates that the signaling path to and from the call server terminates on the same IP address and port on the Oracle® Enterprise Session Border Controller. This means that it’s a spiral call. In addition, the media IP addresses and ports in the SDP indicate that the Oracle® Enterprise Session Border Controller does not need to send the media into the call server’s realm.
An issue occurs when DTMF indication is relayed either by RFC 2833 or DTMF audio tones for a spiral call. Since the DTMF indication is in the media path, the call server remains unaware of the signaling; no media-based DTMF indication digits will ever reach the call server. In order to include the call server in the DTMF-indication signaling, you should set the dtmf-in-audio and/or rfc-2833 mode used for the realm or signaling interface where the call server is to dual.
P-Dual-Info Header
The P-Dual-Info header is used in a spiral call scenario, when the call leg to (and from) the call server is set to dual. While the media portion of a spiral call goes from endpoint to endpoint through the Oracle® Enterprise Session Border Controller, the signaling portion of the call loops through a call server.
The Oracle® Enterprise Session Border Controller inserts a P-Dual-Info header into a SIP info message sent to the call server, which in turn forwards the SIP INFO message back to the Oracle® Enterprise Session Border Controller. Seeing the P-Dual-Info header, the Oracle® Enterprise Session Border Controller knows not to forward this SIP INFO message to the target endpoint because it would be a duplication of DTMF indication already sent to the endpoint in media format.
