[Users] Openser Not Send Ack to Callee?

hi, all: I'm new to here and just installed an instance of openser-1.2.0-notls version under CentOS 4.3 platform. Also I've added mysql db support and it seemed all is working well. But when I made a common call through openser, I found a problem that the openser didn't send ACK message to the callee. The flow is as follows: caller ---INVITE--> openser openser ---INVITE--> callee openser <--180-------- callee caller <----180------ openser openser <----200------- callee caller <----200------ openser caller ------ACK----> openser At this time when the caller send ACK to openser, openser didn't send ACK to the callee. So after 30 seconds, the callee released the call due to time out. Does anyone know what's the problem? Any help will be appreciate!