First, you must have rtp relay on for the sip server and check a conversation to make sure it is working. I assume the sip server is not behind a firewall and the users are right?
My Sip server not behind firewall, I need the system to support firewall traversal to be able for the clients all over the world to call through my Server in two ways.
it's already like that, but still some users can call in one direction only( can call out, but can not receive calls), so there is any additional configuration for this situation.