We have to add support for playing an early announce for those
failed trasnfers, which for example reach the timeout.
This means there is a need to do the following:
- send 183 from early announce DSM module
- receive 183 on the B2B leg, which was sending an INVITE to
the transfer destination
- update the caller with an actual media capabilities,
so it is able to receive the early announcement
The mechanism behind that is that:
- if Proxy in the other leg, which was used to reach the party
(to which the transfer has been done),
requests the force usage of the announcement for the caller
in the very first leg (with the caller who is on hold now)
- then the DSM functionality adds the 'P-Early-Announce: force' header in 183
to let sems-b2b in the very first leg know, that there is a need to embed
the audio from the early media into the media session with the caller.
- hence, caller gets a media re-negotiation
- and the leg with the DSM doesn't get updated, since it's still remains
in the Early stage of the dialog
All the solution is built around two important things:
- Proxy uses the P-App-Param called ';force_early_announce=1' to show to the
DSM that it needs a "forcive" early media, which must "override" the MoH
for the caller, if it's being played.
- Sems-b2b uses the header 'P-Early-Announce' to let the very original leg
with sems-b2b know, that there is a need to embed early media into
the already established media session with a caller
Original ticket number: TT#187351
Change-Id: I2cda231e877d9ba91eaa1738322b0981618c1dbf
Donat Zenichev