FreeBSD 6.0 Analogue Digigium TDM400P FXO Module

Hi there, I seem to have successfully installed my asterisk and zaptel drivers. if I call from my pstn to my asterisk box, that works 100%. However if I call from my SIP phone to the PSTN the call does go through and the phone on the PSTN does ring… however when I answer… I can’t hear anything on either side. what could be the problem? I also don’t get any errors with “asterisk -vvvr” or with “sip debug”