I successfully setup 3cx with all internal extension (entension within the LAN) working well. Got to register also externanal extension but the problem is when these extensions try to call internal extension, there is no audio but we the called party rings. Is there anything that needs to open such as ports from the router? I would appreciate any help. this is the only missing link. The setup is built on Windows XP. Internet connection is through a router (lynksis).