[Asterisk-Users] 403 forbidden on SIP register

[snacktime]

I'm getting 403 forbidden errors when attempting to register to a
certain provider.  I've tried just about every combination of
configuration settings I can think of with no luck.  Following is what
I would think should work (and one of the settings I have tried). 
Rather then list every combinaton I've tried, what are the common
causes of a 403 forbidden on a register attempt?

Other providers work fine using the same syntax.  And the provider
that I am having sip registration problems with works fine with iax.


register => username:password at voip.provider.com/provider-context

[provider]
type = friend
context = provider-context
host = host.provider.com
username = username
secret = password
disallow = all
allow = all