[Users] Relaying and failure route - misrouted ACK

Hi folks, I need an advice regarding failure routes. I want the (Open)SER to route all incoming calls to a PSTN gateway. If that route fails (e.g. because the gateway is down) it shall route the calls to another gateway. I managed to archieve that in the failure case the second gateway receives the call. But the ACK request that the caller sends when receiving the 200 OK is delivered to the primary target again, not to the fallback gateway. My configuration (excerpt): route { ... record_route(); if (loose_route()) { t_relay(); exit; }; t_on_failure("1"); t_relay_udp("192.168.0.37","5060"); } failure_route[1] { append_branch(); t_relay_to_udp("192.168.0.39", "5060"); } Thanks in advance. Regards, Stefan