[Users] ACK ACK ACK

Hello, It's been a little while since I last posted, but this will look familiar to those who remember: The scenario is this: Phone sends openser an invite, which tries to set things up with the PSTN gateway, which is unresponsive(*), and so it sends back to the Phone: SIP/2.0 503 Service not available and the Phone sends back an ACK. What should I do with that ACK? One possibility is to do nothing, another possibility is to t_relay() it. What suprised me was that when I do nothing then another "SIP/2.0 503 Service not available" message is automagically sent to the Phone. So, that can't be the right thing to do. Meanwhile, when I t_relay() it then things usually work. But this doesn't seem like the right thing to do, since the ACK is meant for the very box on which I'm doing the t_relay(). So, what's the story? Thanks, -mark (*)don't worry about why the PSTN gateway is unresponsive.