Stepping out of the session path with an SBC

Hey,

I'm currently looking at different designs for an upgrade of a VoIP network. Currently we have SIP phones registering to Asterisk and then Asterisk placing calls via our PSTN gateway. The gateway handles CDRs, IVR and AAA but slowly as we increase features this is becoming prohibitive. We are also about to expand in to the UK (we are currently in Australia).

Example:
A UK SIP user wants to call an Australian landline phone. The way the current system works they dial 1# which connects them to Asterisk and then transparently to the GW IVR. They then authenticate and dial the number. Now I was thinking of putting an SBC in the UK and one in Oz. When a UK SIP caller dials up the SBC handles the IVR prompts and the user gets authenticates and enters the number. Here's where the problem arises. The number is for Australia so I would rather that the user goes through the Australia SBC and then to the PSTN gateway as this is the shortest path.

However, is it possible for the UK SBC to send a reinvite to the Oz SBC and then drop out of the call? From what I've read, SBCs always stay in the session path. If not, can you suggest an alternative method of a geographically distributed VoIP network?

I'm sure this is clear as mud so please post back if you want more info or anything. Any replies are appreciated. Thanks.