[SR-Users] Dispatcher and "src based" routing algorithm

Hello, I am using kamailio 4.4.3 (x86_64/linux) 5a2195 and Dispatcher module to route calls towards PSTN (4 pstn gateways). The Dispatcher module is using round robin mechanism to select the pstn gateway. I have built a routing logic in a way that when call arrives to Kamailio from one of the PSTN gateways, and fails to route towards an external sip target, the call is marked as CFU call (a diverting call) and routed back to PSTN with different B number, but the actual challenge is that I would like the target PSTN gateway _not_ to be the same PSTN gw from where the call originally came in to Kamailio. This feature is requested because of the technical limitations of the pstn gateway software. So is there any mechanism in Dispatcher or elsewhere in Kamailio to do this? Basically I would need a flag or something to mark the pstn gateway when the call comes in and assure that when routing back to pstn, the same gateway would not be used at all, or it should be used as last target from the four gateways in Dispatcher. Cheers, -- Olli