thanks for your help.
1.When i send an Invite, no matter what responses i got from the server(200,404,415,483), my sip stack automatically sends an ACK, however my program didn't do anything about ACK. in this case, the client can't receive this ACK request. I wonder what's wrong with it??
2.When i work with a real SIP server(VOCAL on Linux) instead of the SIP server Emu, I got everything right before Invite. but when i invite i always got 404 Not Found. I wonder if it is because the two sip stack can't work together or something else wrong?