[Serusers] failure_route question

Hi All. I'm using t_check_status() to do call forwarding on 486 Busy responses. I have an odd thing happening where SER receives the 486 Busy message (for which SER properly sends an ACK response) and then the failure_route calls append_branch() and t_relay() to try another destination. After the new leg of the call is setup and then torn down, the original 486 Busy seems to be hung in the tm module, because it magically appears in ngrep, which shows it being sent back to the caller as if the append_branch() never took place. Is there anything special that I should be doing to "swallow" the 486 Busy response to prevent it from being sent to the caller UA? Regards, Paul