asterisk users - Jun 2006 - exactly what ports are required for sip phone to sip voip connection ?

I can call/receive fine from a zap fxs connected phone to my voip 
provider through asterisk. I can call from a sip softphone extension to 
the zap extension just fine. However using a sip phone extension 
connected to asterisk and calling out through the voip line there is no 
sound.  I was able to resolve the problem temporarily by making the 
asterisk server the DMZ and entirely dropping the firewall.  However 
this seems to be extremely unwise.  I would have thought that the sip 
softphone connected to asterisk inside the firewall then asterisk would 
connect to the voip provider.  What I don't understand is how the 
firewall and port forwarding is different between using the sip 
softphone and the zap fxs port.  So far my Googling has not uncovered 
the secret here.

My next step was to get several sip hard phones like the gxp-2000 but I 
fear that they will work as poorly as the softphones are now.

Any suggestions ?

Mike