asterisk users - Sep 2003 - Problem calling from IAX client to SIP phones

Hello Everyone,

I am trying to configure windows IAX client to call sip client, we are using
grandstream phones. 
I can make calls between sip phones. Also, IAX client can registered to
asterisk box, but not able to call any sip phones.  Anyone knows how the
configuration will be? Where do I have to make the changes? In order to make
this workable. 
It will be very help full. 

Plan is too give windows IAX client to our employees, who are out of office
and can reach one into office calling there direct extensions. Since IAX is
easy to configure into firewall then SIP. 
Windows IAX Client---Internet---FW---*----SIP phones & PSTN GW.
Correct me if I am wrong. 
 
Thanks
Regards,
Kd