[SR-Users] Theoritical Anycast Setup

Hi there, Long time user, 1st time poster - apologies if someone has posted this previously. We have a product from a vendor that utilizes Kamailio as a SIP router - but we're wanting to extend it a little bit by the use of anycast (we're a small ISP). Basically the scenario is that we want to utilize Quagga via BGP to anycast an address, which addresses a glaring load balancing issue with have with people who use only A records. Problem is anycast doesn't work so well in this scenario, just by adding the Anycast as a listen address on the proxy. What we'd want to do is rewrite the SIP so that all responses come back out saying to use the 'real ip'. So I have two variables - say REAL_IP_ADDRESS and ANYCAST_IP_ADDRESS - request will come in via ANYCAST, but I want all responses from then on to be via REAL_IP_ADDRESS. We have six Kamailio boxes in various geographical locations, so the idea is they hit one based on closest, and we rewrite it so they end up with the real ip address to talk to. Anyone done something similar? I'm guessing possibly, but I can't find anything specific enough for me to use. Help appreciated! Regards, Daniel