asterisk users - May 2004 - Echo on sip to sip call

I have a * server that receive calls from the internet interface (eth0,
external) and from the local interface (eth1, internal), when the calls are
between sip phones from the local interface (internal) all is ok, no echo,
but when the call is between a external sip phone and a internal sip phone
there are echo, a delay of 500ms, only on the internal side, the external
side don't hear the echo.

I don't know what I can do, please give me help.

Kind regards,

Miguel