asterisk users - Apr 2005 - Confusion re; 407 Proxy Authentication Required

I am having one hell of a time configuring 2 Asterisk boxes on a VPN
to create a distributed PBX. Pretty much everything is working as I
need it to work, except for one small thing. For some reason, if I
specify a 'secret' for any of the SIP phones on one of the Asterisk
boxes, then I get a "407 proxy authentication required" error when
trying to make a call from that phone.

To put this in perspective, SIP phones on the second Asterisk box can
make and receive calls A-OK. SIP phones on the problematic box (for
.conf files are very similar!) can log in just fine ('sip show peers'
shows that they're registered) but any attempt to make a call gives me
a 407. The funny thing is, these phones are able to receive calls no
problems, it's just the making calls. Additionally, if I remove the
'secret=' line for the relevant extension, then I get no such errors
when trying to make a call.

Me & the 2 Asterisk boxes are all on a VPN. As far as I can tell, the
relevant settings in sip.conf are exactly the same (apart from the
extension numbers) on the 2 boxes, yet they behave so differently. I
have spent quite a few hours trying to debug this and searching
through web sites and mailing lists but just can't figure it out. If
anyone can point me in the right direction, I would greatly appreciate
that!

Regards,
Kevin