I keep an eye on my logs and for as long as I can remember this post has been the most popular: http://www.thecruftofmybrain.com/2006/03/14/cisco-router-with-fxo-as-an-asterisk-gateway/
It's funny that almost 3 years later very little has changed. I'm running Trixbox CE now but the configuration is still pretty much the same. I'm pretty happy with how TB has progressed and I've stuck with it. For a while I was running it virtually and while that worked ok it always had problems with the clock slipping. I'm now running on an older P4 and all's well.
Regarding my old config, a couple of things seem to come up with the folks that find that post. First, you don't want the OPX version of PLAR. I'm not really sure why Asterisk isn't happy with it but it has to do with the order of going off-hook. Just use the regular PLAR and you'll be fine. Also, make sure you put the trunk into the proper context on Asterisk. I say "proper" because everyone will be different.
If someone has a good resource for understanding the proper way to configure contexts, please let me know. I understand the basic nature of it, after all it's similar to Partitions and Calling Search Spaces on CallManager. But it doesn't seem clear to me that there's a simple way to manage them.