How can i get the SBC to use a dfferent source IP address

Hi there

I have a number of clients registering through the SBC, arriving on a realm called access1, these are routed via a Local-policy with a next hop a Session-agent, and realm core1

example below

local-policy
from-address                  
                                *
to-address                    
                                *
source-realm                  
                                Access1
description                    From Access to Core
activate-time                  N/A
deactivate-time                N/A
state                          enabled
policy-priority                none
last-modified-by               admin@x.x.x.x
policy-attribute
     next-hop                       y.y.y.y (SA1)
     realm                          Core1
     action                         none

the issue is the far end (SA1) will only accept a max of 256 registration against the same source IP address (i.e. the IP address the SBC uses for Core1)