asterisk users - Oct 2007 - iax register gets "facility rejected"

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"


Any help appreciated

sean