Try one or more of the following:
1) Run the network setup wizard (in Express Talk). It will help identify open ports and find the best way to route VoIP communications automatically and will solve many network related problems
2) Forward the appropriate ports you are using with Express Talk in your router/firewall configuration (see www.portforward.com)
3) While in the call quickly put the call on hold and then take it off hold and see if this fixes any problems (the hold button has been known to fix many communication issues)