asterisk users - Jan 2008 - Newbie Q: Good link to configuring NAT with Sipura ATA's & hardphones

In trying to get my services up and running, I've encountered the usual 
spate of first-time issues.

I was wondering if there was a good FAQ or Howto on troubleshooting NAT 
issues.

The equipment that I'm using is typically a Sipura ATA or hardphone 
(SPA-942) sitting behind either a Linux box as firewall (using IPtables 
and Arno's firewall) or else a Cisco IOS device running 12.4T and 
Advanced Security featureset (w/ "inspect" configured).  The SIP proxy
is an Asterisk box with a public/routable interface running 1.2 or 1.4 
on Linux.

Usual issues include:

* the first line on port 5060 doesn't register, etc. but devices on 
5061-5080 work fine (caused by IOS interference)
* or else SIP signalling works fine, but there's no media connection.

I've googled various postings on the net, but they're not very 
consistent or even well explained.  Some claim using "VIA" works,
others
espouse using "rport=", etc.

I've looked at the Sipura administration guide, and it's troubleshooting
section covers 3 scenarios (!!!):

* phone won't boot up
* phone won't make or receive calls
* calls with poor voice quality

Ok, well, that leaves a lot of territory uncovered...

I'd be happy to write up a FAQ once someone has explained things to me.

Thanks,

-Philip