freebsd 5.4 with pf nat and voip

[Thomas Maack Nielsen]

I have the following setup:

FreeBSD 5.4 with pf enabled.

pf works as the firewall and does the NAT to.

On the NAT side i got 2 x pc's and 2 x Grandstream 286 voip adaptors.

My NAT rule is the following: 

nat pass on $extern from $intern:network to any -> $extern

This works fine for my pc's, but not for my Grandstreams, they are setup
to use a STUN serve for easy configuration, but the tell me that I am
using symmetric NAT type, and for symmetric NAT a STUN server doesn't
work, is it possiable to change the NAT type to any other than symmetric NAT?
Or is it the STUN server detecting wrong?

Regards,

Thomas Maack Nielsen