[Serusers] append_branch To-tag problem

[Ray Jackson]

Hi all,

We have ser routing in/out calls fine, however we have a problem related 
to the append_branch feature.  After 25 seconds we use append_branch to 
redirect the call off to another number for voicemail etc.  Ser sits 
behind a stateful call routing engine which handles both inbound and 
outbound calls to/from the PSTN.  The problem is that when append_branch 
forks off the call it receives back a different tag on the To: field on 
the OK than that of the original call - hence the OK fails to get back 
to the original callee (gateway complains of non-matching To: fields) 
We've tried stripping the tag using subst on the OK, but then the ACK 
coming back has the same problem (different 'tag' to what is expected in 
the other direction).

Simple question, can Ser work in this situation to 'remember' the To-tag 
  on the forked call somehow - or are we going to need a b2bua to keep 
state and handle the forked call?  Or, perhaps is there a different way 
to redirect a call without using append_branch?  Any help would be 
appreciated...

Cheers,
Ray

PS. I can provide a call flow example if that would help?