Hi,
What does the wireshark trace shows in reference to call flow of leg B. I suspect that when dropcall call is fired, that a BYE is sent? Is there a 200Ok received in this case?
It sound like the transaction timeout is happening because the REFER is expiring since the leg was not dropped.
So check to see what is missing over the wire for the BYE sent and if 200ok received?
Jeff