Failure to connect - COD Private Matches - PS3 PSN

[#1]

Hi, I'm having difficulty connecting to people in my friends list to play private matches on Call of Duty.  I have followed instructions found on the web to put my PS3 in a DMZ through my wireless router - this has worked fine as I can connect to the PSN and play on-line but I still cannot connect to friends for private matches. 
I suspect the problem could with the ports at COD uses?
Any help appreciated.
Cheers
TG

[#2]

if your PS3 is in the DMZ, there is nothing else you can do. the problem lies at their end (you cannot connect to them)
if you tried to host a private game - can they join you? (i would imagine this is the case)