asterisk users - Jul 2006 - Called party cannot hear caller

Hi,
I'm using Asterisk 1.2.1 on a Debian distro. It happens that sometimes 
the caller cannot hear the called party just after the called picks up 
the phone. This happens with inbound calls but also with calls from a 
SIP phone to another SIP phone. Asterisk and all the SIP phones are on 
the same net 192.168.1.x.

Is there anybody who has experienced and solved this problem?

TIA


Giorgio Incantalupo