SIP Registration

[VirginSnow at vfemail.net]

I just signed up with a US48 SIP provider which claims to be
Asterisk-friendly.  However, I had a bit of trouble getting their
service to work with Asterisk.  I am using them for outbound calling
to the PSTN *only*, not for receiving calls from the PSTN.  The
problem I had was this: whenever I tried to place a call, their SIP
proxy reported an authentication error:

WARNING[19327]: chan_sip.c:9865 handle_response_invite: Forbidden - wrong password on authentication for INVITE to '"asterisk" <sip:asterisk at MY_IP_ADDR_HERE>;tag=as2e537c98'

As it turns out, having a "register =>" line in sip.conf is all that
was needed to fix the problem:

register => <<<MY USER>>>:<<<MY PASSWORD>>@<<<HOSTNAME>>>/<<<MY USER>>>

I'm confused why a "register" line would be necessary for
outbound-only service.  According to everything I've read, SIP
registration is used to tell a SIP proxy the IP address of an endpoint
with a dynamic IP address so that calls can be routed *to* it.  Yes,
registration requests do have to be authenticated, but is SIP
registration properly used as an authentication technique?  Or is this
company essentially abusing the SIP registration mechanism?

Interestingly, the provider seems to require *both* registration and
per-call digest authentication.  Missing either one, the outbound call
will fail to authenticate.  This seems a bit bizarre to me.

Here's the response I got from their technical support about this:

> Dear Sir,
> 	I thought I saw a registration string in your config earlier,
> otherwise I would've suggested it at the beginning. 
> 	Registration can be used for inbound or outbound service. We have
> customers that use both and some that use neither. All TalkinIP customers
> are constantly re-registering to verify the server and account information.

Does anyone on this list know if this is how SIP registration is
supposed to be used?  Is this common practice?

After about a week of chatting with the company's tech support, I have
finially gotten outbound calling working with Asterisk.  Now, I'd just
like to know WHY it works...