Windows & MSN Messengers and ZoneAlarm

Jonathan,
We were blissfully using both MSN & Windows Messengers. Then we installed
the ZoneAlarm (ZA) Free Firewall (as a trial), this caused both messengers
to give problems, we were unable to invite other users to a voice
communication but able to accept a contact with users not running ZA.
Switching off ZA resolved the problem.
An enquiry to the ZA Discussion Forum resulted in the following
comment.......

"Voice chats using Microsoft Messengers require the UnPnP ports to be open.
However by default, ZA blocks this port because lots of worms and trojan
horses target these ports too. If you want to voice chat, you would have to
manually open these ports up but that function is only found in ZA Pro. "

Is UnPnP required for Messenger operation?