Remote assistance doesn't work from B to A

> We have 2 PCs in 2 different countries. When the first
> one request Remote Assistance using MSN Messenger 6.1. It
> works fine (with an external UPnP Firewall/Router on the
> XP PRO PC/ADSL and the XP firewall on the XP Home PC/56k)
> But when trying to do the opposite the attempt to
> establish the connection fails. (Even after both
> firewalls have been disabled.)
>
> Any suggestions where to look.
>