asterisk users - Oct 2007 - iax: they hear me, but I don't hear them...iptables?

I'm trying to setup zoiper ( formerly idefisk ) to use my asterisk
server at work from home.

I've setup zoiper for iax, set the ip address to work's fixed ip
address, user: home, password: password

but the zoiper log shows:
11:02:35 Rejected registration  for 'home at my-office-ip-address>'
with
cause 'facility rejected'
11:03:35 Rejected registration  for 'home@<my-office-ip-address>'
with
cause 'facility rejected'

and on the asterisk server at work I get:

 NOTICE[5072]: chan_iax2.c:5252 register_verify:  No registration for
peer 'home' (from <my-home-ip-address>)


iax.conf:

[general]
jitterbuffer=yes
tos=ef
iaxcompat=yes

[iax-from-home]
type=user
allow=gsm
username=home
secret=password
context=long-distance
;;auth=md5,plaintext

[iax-to-home]
type=peer
allow=all
username=home
secret=password
;;auth=md5,plaintext

[guest]
type=user
context=long-distance
callerid="Guest IAX User"

This seems like a NAT problem, but how can that be with iax if they
can hear me? Doesn't everything travel on the same port? How does my
phone ring?

Iny any event, my home iptables -L:

Chain FORWARD (policy ACCEPT)
target     prot opt source               destination
ACCEPT     all  --  192.168.2.0/24       anywhere
ACCEPT     all  --  10.0.0.0/8           anywhere
ACCEPT     udp  --  anywhere             anywhere            udp dpts:1024:65535
ACCEPT     udp  --  anywhere             anywhere            udp
dpt:avt-profile-1
ACCEPT     udp  --  anywhere             anywhere            udp dpt:sip
.................

Any help appreciated

sean