[Asterisk-Users] SIP REFER - find brick wall, bang head, repeat!

[Hendrik Magilsen]

I'm struggling with an implementation of asterisk between two SIP providers.
Provider "A" provides DID numbers to Provider "B", and asterisk sits in the
middle in effectively a back-to-back configuration to try to solve some
compatibility problems between A-B.

So point-to-point everything works as expected

However, the end user at Provider "B" typically wants to do things such as
conference call.  Within Provider "B"'s network conferencing is managed by a
separate SIP conference server.  All parties are transferred to the
conference server and given a bridge reference token.   What appears to be
happening is that the information contained in the REFER sent to the
asterisk (which are connection details for the conference bridge) is not
carried over to the subsequent INVITE.  I believe that when the refer comes
in only the first part of the message which contains the destination
conference at xxx.xxx.xxx.xxx is matched against the dialplan in
extensions.conf, but when the call is made it doesn't carry over any of the
reference information and starts a whole new call tag.

Am I stretching beyond the capabilities of Asterisk in this application?
Should I be looking at a SER implementation instead to get asterisk out of
the middle of a B2B arrangement?  My head hurts so I'm asking the community
at large.

Here is an abbreviated callflow to make it easier to understand

Provider A            Provider B              Asterisk
|                       |                       | 
|                       |                       |
|>F1 INVITE (sdp)------------------------------>| 
|                       |                       |
|<------------------------------- Trying 100 F2<| 
|                       |                       |
|                       |<------(sdp) INVITE F3<| 
|                       |                       |
|                       |>F4 100 Trying ------->| 
|                       |                       |
|                       |>F8 180 Ringing ------>| 
|                       |                       |
|<------------------------------ Ringing 180 F9<| 
|                       |                       |
|                       |>F10 200 OK (sdp)----->| 
|                       |                       |
|                       |<------------- ACK F11<| 
|                       |                       |
|<-----------------------------(sdp) OK 200 F12<| 
|                       |                       |
|>F13 ACK ------------------------------------->| 
|                       |                       |
|                       |                       |
|                       |>F23 REFER ----------->| 
|                       |                       |
|                       |<---- Accepted 202 F24<| 
|                       |                       |
|                       |<-(sipfrag) NOTIFY F25<| 
|                       |                       |
|                       |<------------- BYE F26<| 
|                       |                       |
|                       |<-----(sdp) INVITE F27<| 
|                       |                       |
|                       |>F28 100 Trying ------>| 
|                       |                       |
|                       |>F29 100 Trying ------>| 
|                       |                       |
|                       |>F30 200 OK ---------->| 
|                       |                       |
|                       |>F31 404 Not Found --->| 
|                       |                       |
|                       |<------------- ACK F32<| 
|                       |                       |
|                       |>F33 BYE ------------->| 
|                       |                       |
|                       |<---------- OK 200 F34<| 
|                       |                       |
|<------------------------------------- BYE F35<| 
|                       |                       |
|>F36 200 OK ---------------------------------->| 

Thanks in advance
Hendrik