Just tried out the $70
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ATA I (thought I) saw reccomended here with my Asterisk@Home 2.8 PBX, and it worked flawlessly. A bit of initial difficulty setting it up was due to the lack of supplied documentation (a single piece of paper with "Here's a hidden pointer to the manual on our WWWebsite"), but getting it up and running was almost trivial. Fired it up on my LAN, found it's IP (push the button and pick up the phone and it speaks it's IP!), pointed my WWWeb browser at it, configured it to work on the LAN, made calls back and forth (G.711 and GSM were the only compatable codecs, but GSM is perfect for what I want), pointed it at my WAN IP, put it outside the firewall, and it made calls flawlessly. Can't wait to try it out far from home!
I think the hardest part was figuring out how to poke a hole in my firewall at port 4569. 8*)
Anyway, just a happy customer, in case anyone else is looking for a similar solution.