[SR-Users] Copy reason field from 503 to 500.

Hi all, I have Kamailio connected to PSTN gateway. Sometimes gateway replies with "503 temporary unavailable". PSTN gateway adds a "reason" header with ISUP code. But according to RFC 3261 kamailio doesn't forward 503 replies transparently. When 503 reply is received Kamailio replaces 503 with a 500 which is created from original request (build_res_buf_from_sip_req). I have tried append_hf from textops module, but it's not working with a locally generated replies. How can i copy Reason header from 503 to 500 ? I will appreciate your help with this situation. Best regards, Vitaliy Aleksandrov.