[Serusers] cancel invite

Hi, Happy new year to all! I run into one case that ser doesn't cancel pending INVITE after CANCEL is received. When a caller sends INVITE to ser, ser forwards to the contact location in its record. If a contact is not expired, but the phone is not available somehow, ser keeps trying to send INVITE. During this time, the caller gets no response because no one sends 180/183. At this time, the caller hangs up and sends a CANCEL. Ser sends back "SIP/2.0 200 ok -- no more pending branches". The problem is that ser still keeps sending INVITE which should be cancelled by the CANCEL. In some cases, these INVITE eventually reaches the phone. The callee phone starts ringing even though the caller has hanged up already. Is there any fix to the problem? Thanks, Richard