asterisk users - Apr 2012 - Peer SIP authentication with Taqua switch

I'm using Asterisk 1.8.6.0 on my router talking to my ISP's Taqua 7000
(?) switch.

I'm using a config that looks like:

[sip_proxy-out]
type=peer
authuser=208nnnnnnnn
remotesecret=xyzzy
qualify=100
host=n.n.n.n
call-limit=5
nat=no
; sendrpid=yes
insecure=no


But the Taqua responds to outbound INVITES with 403 Forbidden (oddly, not 401 or
407).

Also, from what I can tell, the outbound INVITE doesn't seem to have any
fields that would imply authentication (unless console SIP debugging strips
sensitive fields from output).

What am I missing?

Is there a good configuration and/or troubleshooting guide?

I looked on voip-info, etc. but most of that covers client/server
configurations, not trunk peering.

Anyone have a config that they managed to get working?

Thanks,

-Philip