asterisk users - Feb 2007 - Asterisk behind OpenSER - Getting SIP reinvites to work with an ITSP

I'm using Asterisk (1.2.14, RedHat 9) but I've been having trouble with
SIP
re-invites.

I have a DiD from an ITSP and when someone calls in, Asterisk plays a menu
recording and transfers the call to the external line the caller selects.
Since both sides of the call are external, I want to use re-invite to avoid
the rtp packets from going through my server after the call is bridged.

I have gotten this to work flawlessly with IAX but not with SIP.  It appears
to be a NAT problem.  BTW - I avoid any options in the Dial command that
could keep Asterisk in the way e.g. Dial(SIP/<external
number>@<myITSP>)

My readings led me to SER, and since I want to avoid a NAT, having SER on my
Internet gateway makes most sense, which led me to Milkfish on a Linksys
WRT54G.

Having never worked with this before I wasn't sure how it would be
configured in the end, so I just "went for it".  I assumed that
OpenSER
would sit between Asteisk and my ISTP and handle both ends of the
resgistrtion.  I was able to register two Xlite phones to my WRT54G and have
them talk.  Not so lucky w/ Asterisk.

I was able to create an account on OpenSER for my DiD and have Asterisk
register on the router, but how would the ITSP end know where I am?  I was
hopeful that I could simply continue to have the register in sip.conf go
directly to my ITSP and OpenSER would just "take care of it" and
silently
manage both sides of the registration similar to what happens on some
commercial devices like InterTex's FireBird, but no luck.

Can this be done?  If so, what changes do I need to make to sip.conf etc.?
If not, can you suggest an alternative so I can get the reinvites to work?

In sip.conf I'll obviously need to make nat=no and canreinvite=yes.  I
don't
think I need the following lines either because I'm no longer behind a NAT:

externip= <my external IP> ;Outside address
localnet=192.168.0.148/255.255.255.0 ;Inside Network

The registration has me perplexed though:

register => <my DiD number>:<secret>@<myITSP>/<alias>

How does the ITSP end know where I am?

I'm grateful for any help/direction.

Thanks,
Hugh
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