By default, SEMSi [1] forgets a session as soon as a BYE is sent. To suppress error messages on unhandled replies to appear in the log, use the unhandled_reply_loglevel configuration option.

Explanation: In short, the UA layer (AmSession) by default forgets the dialogi [2]/session as soon as the BYE is sent, thus the transactioni [3] layer can not deliver replies to the BYE. The question is what a proper handling of negative reply to BYE would be - the only exception with this could be if a BYE needs to be authenticated (401/407 reply), but i have never seen this in reality. If you need it, you can code it in your application though (don't call setStopped when sending BYE, catch replies to BYE cseq in your onSipReply, and act on that properly, e.g. call setStopped only when positive reply received or severe timeout...).