It is still not working.
I think DMZ is a superset of port-fordwarding. So if I already use DMZ, then port-fordwarding is redundant.
If the caller is calling from a registered SIP account, say a CallCentric account, then the call can connect ( as verified from the caller's screen ), but there is no audio afterwards.
If the caller himself is also calling from an un-registered account, like from another OBI box, with the same set-up as that of mine, then the call cannot even connect, let alone audio.