asterisk users - May 2004 - Help ! Echo on sip call.

I have the * server in a machine with two interfaces one internal (eth1) and
another external (eth0), I'm in the internal side and when I put a call to
another internal sip phone all is ok, but when I put a call to a external
sip phone I hear the echo of my voice, a delay of 500ms.

Is here anyone that knows why ?

Thank's a lot.

Miguel