Thanks for reply.
I use ADSL modem connection in both side. one side setup a gatekeeper (GNUGK). Gatekeeper is site behind NAT. I forwarded 1719-1721 ports to the internal address that used by gatekeeper. another side, I use a router to do the same things (forward all ports to the internal that the phone using). some times the connection has problem. because I need to install many remote site that connect to the gatekeeper (main site). some site use the same connection but with problem. It can connect to the gatekeeper and can ring. but no voice can be hear. some times one side and some times both side. So I need a tools to check what happened in those site. I am sure that the voice codec are the same. The call path is:
endpoint –gatekeeper–router–ADSL modem—Internet—ADSL modem—Router—endpoint.