[Serusers] append_branch To-tag problem

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?