MSN messenger - unable to use phone feature

I use a cable modem(RCA) with router (Lynksys BEFSR41) to
connect to the internet.

when I try to use the phone feature from MSN Messenger
6.2, it gives an error message "Your network is
configured such that you cannot make phone calls"

I enabled the UPnP services in the router configuration
(as per the troubleshooting guide) and restarted the
messenger engine. I'm still getting the same message when
I try to use the phone feature.

Any ideas?

thks